I don't know exactly. I'm 36 weeks and my LO has only just slowed down but I think for some they run our of space much sooner. I still get a foot poking out every now and then and I get ripples of movement like he's wriggling a bit. Last week I had a foot stick out my side so hard I thought he might actually break out of my skin.
If you're worried your LO has slowed down a lot then I'd call your midwife.
