Same as when starting with foundation, although it usually happens faster.
When the bees are using at least 80% of the frames for storage or brood add the next box. If you add it too fast you will end up with undrawn foundation or unused comb in the bottom box, and this will give pests too much room to operate, not a good thing.
Note that just having bees walking around on the comb is not the same as bees using the comb, it really must be full of stores or brood. Don't wait until you have bees hanging off the front of the hive, but wait until the hive looks crowded, that way they will build out your foundation right away.
Early is probably better than late on adding though, especially if you have only foundation.
Peter