I say I made Filler in around a month back in 2008, but really I made it one night--the prototype, anyway. From there it took a couple of nights & weekends to add some menus, ads, and high scores. I'd say I've booked somewhere in the neighborhood of $50-$75k off of it since then, and it still makes decent ad money today.

A weekend is more than enough time to get a prototype together that's worth devoting more time to. Ludum Dare is an excuse to put aside the rest of your life for that weekend and take a crack at it (especially for people who don't normally make games).