You can input them as fast or as slow as you want, you just have to make sure that you key in the sequence correctly. To do Pummel, select Sabin's Blitz command in battle, then press "Left" "Right" "Left" on the control pad (the directions, not the shoulder buttons) and then confirm it with your confirm button ("X" or whatever). If you do it right he'll do a Pummel, if not it'll say something like "Incorrect Blitz Command at the top of the screen.

EDIT: Bah, beat by two other smarty pantses.