I like to change my round wounds evey 2-3 weeks. I like the bright slap pop sound. I would say it's almost impossible to get a good slap tone with strings older than a month or two.
I would bet that you won't catch slappers like flea or a tapper like claypool playing on a two month old set of strings.
Flat wounds on say a fretless are a different story. They don't seem to sound much different after several months. I'm sure some of the avid fretless players would dissagree though.
If you're trying to get a flea type sound, flatwounds just aren't going to sound right.
I have seen guys spend thousands on bass guitars that when I pick them up they have old dead round wound strings. To me they might as well have a $99.00 Squire and spend the extra money on regular string changes they would probably get a better tone (in my opinion).
