If it is a fairly sorted and well maintained car, I would say you it isn't that much. Unfortunately, I am a bad example because I am missing driving FM at nationals due to car issues. But I ran over a year (probably a good 16 or 18 events) with a co-driver without missing a single run due to issues. We just ran into some things due to the design of the car and our need to go to shorter gearing, convert to chain drive, etc and then some fuel delivery issues this past weekend... but other than that, the car has ran flawlessly.
Main things are to probably clean the carbs once in a while. The clutches need cleaned more often probably. We have been bad about our maintenance on the clutches, but to be honest, you can tear them down, clean them, and re-lubricate them in an hour. And I'd say maybe do this every 4th event or so and before big events.
Of course, if you are seeking every last speed tweak, like dealing with tuning the clutches, and running the jetting on the ragged edge, etc.... there are lots of things to try and that can be time consuming. But if you are happy with a car that is close and can get you into the trophies with the right driver, then there isn't that much to do.
Oh yeah, and they are an absolute blast to drive. It more than makes up for the fact that everyone hates you for running a two stroke engine that blows smoke everywhere and revs to 8k and stays there.
For more opinions, try posting on formula500.org or f500.us....hope this helps.