Suprisingly long. I've played around with that a bit a while ago and sometimes up to minutes later you could still recover recognizable bitmaps. But definitely not something you should rely on if you want a bit perfect copy, the first bits start to flip immediately upon power loss, some of them take a lot longer and our brain is pretty good at such reconstruction from noisy data, especially if it knows what it is looking at.

I guess the higher the RAM capacity the shorter it would be because of the decrease in physical cell size.