It really depends on how good I believe the new new material to be and how familiar I am with the band's catalog in general, and it is different for every circumstance.
When Queensryche toured on Promised Land, I was excited that the whole album was performed that night.
When Def Leppard toured for Songs from the Sparkle Lounge, I was disappointed that only one new song was played, as I thought it was a decent album and was worthy of more exposure.
When I saw Queensryche's opening show on the current 30th Anniversary tour, I was very happy only two songs were played from DTC.
As I have generally not enjoyed DT songs released post-SFAM (with the exception of ToT live), I have been generally disappointed with the last few DT shows I have gone to because there has been a strong emphasis on newer material. If I connect with the forthcoming album, I hope that I will hear a good chunk of it.
Iron Maiden's Brave New World tour featured 5 or 6 new songs, all the ones I wanted to hear (with the exception of Out of the Silent Planet), so I had no complaints. On the flip side, although I didn't see it, I am sure people were pissed when they did the entire AMoLaD album on that tour.
When Rush played the majority of Snakes & Arrows, I had no complaints as I thought it was a good album and the songs translated well into the live setting.
When I see someone like John Mellancamp, I only want to hear radio hits.