Lean individuals trying to get rid of that last stubborn body fat - or at least more normal/healthy levels of body fat
Athletes who are on a ketogenic diet and need to replenish glycogen stores regularly
If you are on a very low calorie diet and also exercising a lot (although that combo isn't recommended)

Who is it not for?

If you are obese or very overweight then you typically don't need a carb feed but you could do it if you feel it is helpful. See "How Often" below.
If you are insulin resistant or generally feel better on a ketogenic or very low carb diet, then a carb feed might not work out as well. However, there is no harm in giving it a test. See suggestions below.
If carbs are trigger foods, you may want to avoid (see Avoid Trigger Foods)

Your weight loss and diet should improve after a carb feed. You should also feel better overall than before the carb feed. If not ... or if you regain weight that doesn't come off, or if it is extra hard to return to your diet plan ... then obviously carb cycling may not be for you.

How Long is the Carb Feed Period?

A short protocol may only be 5 hours on a single day (for example, from around 4pm to 9pm). Or from breakfast to dinner. Longer protocols last up to 3 days. An average might be 12 to 24 hours.

The shorter periods of 1 day or less might not be long enough to impact metabolic rate, hormones or appetite (although some studies in rats show short carb feeds do impact metabolic rate). However, it is still beneficial because it refills muscle glycogen for better workouts (important for high-intensity exercise like weight training, or endurance exercise) and because it turns off catabolism, allowing muscle building. Perhaps even more important, it give you a psychological break. You can eat those carbs you've been craving.

Also, if you took a full 3 - 4 days of overeating carbs, there is a risk of putting on extra fat. So generally, the safest route for the average person (as opposed to a dedicated bodybuilder, for example) is to keep carb feeds on the shorter side. At least to start. If they go well, then you may test longer periods.

It's generally best to start with a short carb feed first to test the effect, no matter how metabolically flexible you are.

How Often?

Generally speaking, the more body fat you have, the less frequent you should do a carb refeed.

Remember that the you are adjusting calories via carbohydrates. Typically you restrict carbs during low-calorie days, and increase them on the high-calorie or carb days.

5:2 Cycle
Eat low-calorie/lower-carb for 5 days, high-carb for 2 days. The 2 days are usually on a weekend.

3:1 Cycle
Eat low-calorie/lower-carb for 3 days, high-carb for 1 day. This seems to work well for people with lower body fat levels.

Timed with Workouts
Eat carbs only immediately before, during and after workouts -- especially afterwards, as exercise temporarily elevates insulin sensitivity to high levels. This is a common recommendation among bodybuilders and other serious athletes. You don't eat quite as much on this schedule because it's more frequent.

Combination of Flex Meals and Carb Feed
You can play with different scenarios. For example, a carb feed mid-week, and a Flex Meal on the weekend.

As Needed
Some people are in tune enough with their body they can sense when it's time to do a carb feed. You might just feel hungry. You might sense that things are "shutting down" (metabolic rate, sex drive, hunger, etc... all the indicators of the body fighting back). Try a carb feed one day, then another day, until you aren't so constantly hungry. And you do this as often as needed ... weekly or every other week or whenever it occurs.

Eating Guidelines

You will be boosting calories by eating more carbohydrates to at least maintenance levels. Typically it is higher (overfeeding) so you are in a calorie surplus or "positive energy balance".

This means watching fat intake carefully. Otherwise you will gain too much fat.

Keep fat intake below 50 grams per day of the feed
Track and measure the fat
If on a high fat diet, you will need to lower your intake to 50g or below
If you are on a low fat diet, you can increase your fat intake if desired... but not above the 50g limit

Be sure to keep protein intake at your normal level.

Other suggestions:

It's best to retain a healthy meal base (the usual: protein, vegetables, and perhaps fruit). See Standardized Meals. Then add carbohydrates on top of it.
Whole food carbs are ideal, but ... for greatest psychological benefit this is a time you could indulge and handle more refined carbs.
Try to limit sucrose to 100 grams.
The rest of your intake should come from complex carbs/starches (like breads, pasta, potatoes, oatmeal, etc (see Carb Type Strategy)

If your eating period covers an entire day or multiple days, try to eat every 2 - 3 hours -- this helps you avoid energy crashes and is ideal for upregulating hormones.

Other recommendations are similar to Flex Meals:

Do a carb feed on a weekend or during a holiday or social setting
Don't keep foods around house beforehand -- make a special shopping trip for them and buy exactly what you need to eat

How Much Carbs?

It depends on your body fat level. A very lean person trying to get extra lean will need (and/or can handle) more carbs. An obese or very overweight person should either not be doing a carb refeed or needs to use minimal amounts.

However, a very general and simple guideline for high carb days (refeed days) is to eat 50% more calories than your normal maintenance levels. So if you maintain weight at 2000 calories, then you would aim for 3000 calories -- again, mostly from increased carb intake. Of course if you do it too often (like 3 or more times per week) then you probably won't end up in a calorie deficit which means you won't lose weight.

Here are some suggestions for carb intake based on refeed length. Lean individuals should use the higher number in the range provided. Overweight should use the lower.

5-hour refeed: 1.5 to 3 grams per pound LBM (~3-6 g/kg)
1-day refeed: 4-6 grams/pound LBM (~8-12 g/kg)
2-day refeed: First day 4-6 grams/pound LBM (~8-12 g/kg); Second day 2-3 grams/pound (~4-6 g/kg)

Other Suggestions

A carb feed can give you a psychological boost and for that reason it can be a time to indulge a bit in the more refined carbs and even sugar.

However, if you are insulin resistant or don't do well on carbs at all, you may want to limit types of carbs to the Whole Food Carbohydrates if more refined carbs give you trouble (see Carb Type Strategy page for guidance). The other suggestions below can also help.

Also consider:

Get at least a little "magic" in your meal via adequate protein, fiber, and a little fat (see Magic Meals)
Eat carbs only immediately before, during and after workouts -- especially afterwards, as exercise temporarily elevates insulin sensitivity to high levels. This is a common recommendation among bodybuilders and other serious athletes.
It also helps if your glycogen stores are depleted from exercise, to avoid fat regain.

Weight Gain

Because you are eating carbs, you should expect to gain some water weight. This is normal. It's mostly water weight and it'll come off rapidly over the next couple of days after the carb feed.