How often do you change your manual transmission fluid?

[Peakster]

I drove to Wal-Mart today seeking my usual $24.71 Tech 2000 oil change for the Geo, only to be informed it was going to be a 3 hour wait (which was weird because it was after 3pm and the auto service department closes at 5pm).

I ended up going to one of those 'drop in' oil change places and paid nearly $41 for the base 5W30 oil change. Everything under the hood was inspected fine, except for my manual transmission fluid which looked like a dark grey soup. The oil change person said it would be $49.99 to change it, but I declined simply because I didn't want to spend the money today.

Is there places that can do this manual transmission fluid change for cheap (Wal-Mart has always put N/A next to trnsmission fluid check)? How important is it for this fluid to be changed?