To be or not to be that is the question:
When to add is easy, how long it will take for a given colony to fill a hive body of foundation is any ones guess. If you have a real strong queen and they build up very quickly this could be in as little as a week, or it coould go as much a a month. The only real way to know is to look at them over a period of time, then you will get to know what hive is doing what. This is the main point of keeping records, if you write it down ( every time whith out fail) you can get a real good idea of when to do or not to do any given managment.
as a beginner i always and very strongly recoment a boardman feeder, becouse of inspection and feeding loss. You can just look at how much they are eating and from there get a very very good idea of how much work they are doing without having to open up anything. secondly less deisruption when filling the feeder, just rome it in the venening and replace it, when they get up moving the next day it's full and eady for them. third, if you have to medicate them you can with out having to empty out and refill, it is simple to just remove the boardman and place a second jar with the med's in its place. Th KISS methoed is the best