If he wins his 8th world title, he should retire IMO. That way he will retire as a champion, and thats the way to leave the sport. And its not like once he's gone, its all over. Im pretty sure Raikonnen will take his place and him and Alonso will duel it out next season. Schumacher will not retire disgracefully like say Villeneuve did or previous champions like Damon Hill and etc who were just not competitive. Schmacher has pretty much shown everyone the bar, and no one can top it. He will be remembered along the lines of Senna, even better then Senna.
I will truely be sad once he's gone, I've looked up to him ever since I was 9 years old, he was and still is my idol!