When should I drink protein powder if I follow intermittent fasting?

If you follow intermittent fasting strictly, then you would want to include your protein powder within your eating window. For example, if you typically only eat for 8 hours during the day, then you would include it somewhere within that 8-hour time frame. Normally, people following intermittent fasting don't consume any calories in their fasting window, and all protein powders will contain a significant amount of calories. 

Within your eating window, popular options to include your protein are after workouts, in between meals, part of a meal, etc.