A good rule of thumb is the first year is do not harvest and come spring you should have a good idea how many you can take by what was left that they didn't use. Then come harvest time you will have a general idea how many you can take from there on. Just remember its always a good ideal to harvest conservatively.It's always a good thing to leave more than you think they will need you never know they just might need it after all. But having said all that if the hive is getting full harvest a couple of combs this will give them something to do and you a little bit of that liquid gold we all love. especially if it is early in the season they could get to feel crowded and set up to swarm in that case take the empty bars and put them next to the brood nest this will give the bees the impression they have room to grow . But if it is getting towards fall try to be conservative.