The Science of Setlists (or Your Fans Own Your Albums, Idiot!)

I feel it is time to once and for all lay down the law on the proper ratios and key ingredients of a great setlist at any point in a bands career. So assuming for simplicity a rock and roll band with 10-12ish 3-5ish minute songs per album here is what you should do:

After 1 Album:
Play 15 songs.
No matter if your album has 9 or 17 songs on the album play 15 songs and give people their money's worth. Covers, b-sides, unreleased songs it doesn't matter, give your people 15 songs.

After 2 Albums :
20ish songs. 60% of the set from album #2 and 40% from album #1.
Most bands want to showcase how great their new album is and are tired of their first album at this point anyway. But your fans likely still love the first album, so don't leave them out in the cold. By the same token if you're only playing 3 or 4 song from your new album at this point, you're already coasting on the past.

After 3 Albums:
22-25ish songs. 50% from the new album, 50% from the first two albums.
Now that you have more leeway you don't have to make their set nearly as heavy with new stuff, but they should still be playing plenty from all three earlier albums.

After 4 Albums:
25ish songs tops. 40% new album, 60% from the first three.
At this point your set will be as long as it should ever get, but don't drag things out. If you play much more than two dozen songs it just means that more fans will need knee surgery and chiropractor visits.

After 5+ Albums
Still 25ish songs. 30% new album. Make sure to play songs from every album.
Unless you have 15 albums, you should still be touching on every era. Maybe favor one or two of your old albums, but keep things mixed up.

Other rules:

Go Easy On the Unreleased Material
Even if a band is really excited about songs that haven't been recorded yet, there's still no way they should be playing more than three or so.

Always Play At Least Five Album Tracks
If you have a large following of avid fans, then they own your album! Sure some audience members would like to hear the hits, but the album buying public (even as it shrinks) is the population that will keep your career going.

One Encore is Generally Enough
Unless you played the best show of the tour, or you just finished a great hometown gig, one encore is plenty. Only a select few bands like Wilco have established a great reputation for multiple encores, and that was only after they had a good amount of material to work with.