I was up to level 45 at one point but down to level 42 before I quit. Donated 1600 grains of rice for my effort. Made me nostalgic for studying for the SAT and GRE... (yes, I'm weird, but I also got 760 out of 800 on the verbal portion of the GRE... back in '91)
I challenge you to beat my score :-) (Not sure exactly how long I spent on it, but probably less than 15 minutes, and I didn't cheat by looking up any words...)
Edit: They have Spanish vocabulary and a bunch of other subjects too. I got up to level 10 on Spanish quickly and donated another 1700 grains before the words started repeating too often, as there are a lot fewer words and levels than in the English vocab.