I have 18 month old twins, still in Grobags - I love them!
My son is a giant, and his feet are pushing to the end of his. His twin sister is quite a bit more petite, still lots of room to grow. I was also contemplating - when to switch to regular covers? I thought upcoming summer might be a good time, but from the sounds of the other replies it seems kids can't really keep the covers on for some time to come! I think I'll wait.
I am pretty sure they make an 18-24 month size of the Grobags (expensive, yes). But the investment might seem worthy when you get to avoid many a 4 am waking!
Val