Shannon,
Typically, I've driven a circuit [100s of laps] until I've reached a base line in consistancy [know the circuit] and think I know the best racing line on all the corners using the Default setup provided.
Then I've gone into the setups and made small changes and returned to track for another 5 laps to test.
So in and out of the garage for the whole evening .... weekend even ... but I've learned some stuff I think [at least for me] and it's horses for courses I suppose.
I also use forum posted setups for comparision and when I find a good one [ as in an improvement on what I have] I incorporate it and I work to fine tune that.
I use motec to compare laps [speed carried through the corner etc] and focus mostly on my weak points ... eg. Zandvoort turns 10 & 11 in a DTM 2010 car.
Patience, testing, practice, good luck and success!