Got to 666 (level 7) and wanted to stop, so I did. Of course, the problem is that when, as in this case, there are only two columns remaining, that limits how many letters it can throw at you at once and it actually becomes easier to keep up - you can theoretically go on as long as you want.
Maybe it also needs to be clearer that the key closest to the bottom of the screen must be typed first, that wasn't obvious to me immediately.
I think it's good to try random letter games like this occasionally as well as regular typing tests.