So well done Xar!There are 50 levels in all, but it is rare for people to get above level 48.
It's apparently three words correct->up a level; one word wrong->down a level. So you need to get a lot of consecutive words right to move up, and they get harder as you go.
Edit: I deliberately got a lot of them wrong so I could work my way up from level one. I should do it more often: some of the high-twenties and 30-40 words were ones I've not seen before. If I play normally, I start the game at 40 every time and miss out on learning those.