For what it's worth.

When I was singing regularly in coffeehouses two or three nights a week (and I did that sort of thing for a number of years), I generally sang four or five 35 or 40 minute sets per evening, and I tried to avoid singing the same song twice in the same evening—or over the same weekend—unless by request. Nor would I sing the same set two weekends in a row, although I may repeat some songs on consecutive weekends.

As a "set planning kit," I got myself some packs of 3 x 5 file cards and wrote the name of one song (along with other info, such as what key it did the song in, where I learned it, and a bit of background on it) on each card. From time to time I would take a thick stack of cards, shuffle them, and deal out about a dozen or so cards. I would then go through the cards, put some in a "yes" pile and the others in a "no" pile. Then I would deal out some more and do the same. When I had about 40 minutes worth of songs in the "yes" pile that were varying in key (avoid singing two songs in a row in the same key) and varying in tempo. AND varying in mood (not all funny stuff and not all downers), AND varying in length (some short, some a bit longer, from time to time a long ballad). Usually they varied in subject as well, but I would also construct sets of all sea songs or all children's songs, or whatever. Thematic.

I would then write the set list into a little notebook that I could carry around in a shirt pocket. I usually had a couple dozen sets all planned. There were some song repeats, but not many.

I think I knew about 200 or so songs at the time, and by very rarely repeating a set exactly, over a time I developed a reputation for knowing "thousands of songs!" (Not quite! But whenever I heard someone say it, I would just smile enigmantically.)

This made concert planning a lot easier too. Figuring a 50 minute first half and a 45 minute second half, with, hopefully, a few encores, I could take a couple of my preplanned sets, maybe add a few songs, and switch a few around, I was all ready to go.

And in the course of laying out the sets, in terms of length, be sure to figure in enough time to blab a bit between songs—but don't overdo it! Know roughly what you're going to say, but don't repeat the same spiel verbatim.

I once heard Richard Dyer-Bennet sing three concerts on three consecutive evenings. No repeats. I asked him how many songs he knew, and he said that he estimated that he knew about 700. Then I asked him how he managed to keep that many songs fresh. He responded that, on a concert tour, rather than repeat the same concert in every city like some performers do, he tried not to repeat a song until he had gone through his whole list. He used a system similar to mine.

The "shuffle and deal" system also works well for programming records or CDs. There, you really shouldn't repeat yourself. Unless there is a darned good reason for it, don't repeat songs on subsequent CDs.

Don Firth