The reason they are doing well is because they have less light than they'd expect, ironically enough. They are furiously trying to lay down new tissue so that they have more to photosynthesise with.
Keep them like that forever and they'll eventually suffer. But if you put them out in the sun in spring, they'll mature even faster because of all the foliage they have grown indoors.
Put them back under lights next winter, skipping their dormancy, and you'll have mature plants in 2-3 years.