My answer is - it depends. In general, I am good for one show per tour per band, even if they are a band that changes the setlist from night to night. If it is a band I am into, I will make exceptions when it comes to bands that change nightly.
For band that have static setlists, it really depends on whether there is something unique that I feel I likely won't get a chance to hear again. For example, Rush's Time Machine Tour. That tour likely represented the only time I will ever hear "The Camera Eye" live, so I saw both shows that came to the DC/Baltimore area. I took in two shows on Rush's tour supporting Test For Echo because of the performance of the entire 2112 song.. I took in two shows during the first American leg of DT's Metropolis 2000 tour because I guessed that SFAM would never again be played in its entirety following that tour. Same with Roger Waters' recent tour for The Wall, Coheed & Cambria's recent Neverender shows when they did all of IKS. I would have seen DT twice on this tour to hear Space Dye Vest a second time but helping a friend in need kept me to only one show.