User experience makes questions like this tricky: graphics, sound, score-tracking, tightening up controls. If I was working with a graphics library I was already familiar with, I doubt the core gameplay would take me more than a few hours. Writing a reasonable clone of the version of Tetris I played fifteen years ago on the Atari ST would take at least a couple of weeks, though, and that's if I had all the resources I needed on hand. An exact clone would take even longer.