The line counter stops at 999, but since you have to clear ten lines to advance a level, I cleared about 2,080 lines. I beat Nix’s record of 4,609,116 (around 1,660 lines), but only because she had to go to bed and her husband wouldn’t let her keep the DS on overnight. (The objective of the game is to clear 200 lines, but once you’ve done that you get the option to play on endless mode.)
Playing at Level 20 speed lulls me into a trance-like state. Maybe if I play Tetris long enough I will reach enlightenment and the secrets of the universe will be revealed to me.