And the answer is... the most of the times. I often attend to gigs where I leave the place when the headliners start playing. Examples? Miolo remarked one of them. And this won't be the first time I should endure the painful and evil touch of Stratovarius, 5-6 years ago I attended to the Stratovarius + Angra gig just to see the second one. Or Pink Cream 69 + Axxis, when I came to see a 30 minutes Axxis show.
So, if we talk about gigs to see just the headliners, I can assure that there were not more than a reduced bunch of gigs I attended to because of the whole show.
My brain is offline, I apologize for any inconvenience.