In each case, you use as much as you need.
Sorry, but that's the best I can do with limited information. But let me try to give you some useful areas to consider:
Your solvent is gone from the rotovap when its all gone. How much did you put into your reaction? How much total volume did you lose in workup? You can use those values to ballpark how much the rotovap is to remove. Only you know these things. And you're expected to keep track of them, for steps such as these, and general laboratory hygiene. Consider: suppose you do this process 20 times a week, or you are promoted, and you and two lab techs do this 50 times a week -- how much solvent does the lab manager need to buy? How much solvent waste do you need to contract for disposal? I know, you're still in school, but you should start keeping track of what you're doing.
You need enough drying agent to remove all the moisture. Its hard to know exactly how much there is, but if you've evaporated much of the solvent, you can only have as much moisture as is miscible in the remaining volume of solvent.
If you have to wash a product, you have to use enough water. How much is enough depends on how much there is. Only you know how much you added or produced. You probably want to wash 3 times, with 1/3 the volume each time, for best extraction.
Generally, you want to be generous at each step, doing slightly more, so your ballpark figure can be as loose as you need to be to save time. And of course, only you know if any of these procedures is detrimental to your product, in which case, it is better to do less.
Clear as mud now, right?