So far, I've been able to stand through 7.3m, but that was a long game and I started getting tired from standing. I definitely need to get a stool, but playing Robotron sitting down is difficult since I tend to move my entire body playing this game. I'm definitely not a marathon player, I know this now.
Which is why I either play till I hit a couple million, or change my setting to Twin Galaxies Tournament Settings which are quite fun: Default difficulty, 5 lives only, no extra lives...