I am working on arranging a large orchestral work (~45 minutes) for solo bass trombone with orchestra (and a piano version as well). My plan is to make it more of a "selections from ..." type of work and trim down a lot of the material, especially portions where the soloist would excessively large periods of rest. My estimation is I will probably land somewhere between 20 and 25 minutes. I don't think that duration is unreasonable, but I'd like some outside perspective. I welcome any thoughts you all might have.

25 minutes is on the long end for a concerto these days. There are some that go up to 40 minutes or so, but that's rare, especially in the brass world, and typically worked out well beforehand (like years in advance) with an orchestra.

Do you have a performance lined up, or any opportunities to present this piece? If so, reach out to the presenting organization and soloist and have clear expectations set up on all ends.

30 minutes is almost beyond the limit of what an average person can listen to without clapping, these days. I haven't heard a trombone concerto that went longer than that, either, and each time it was 25-30 minutes, the soloist was either Joe Alessi, or Christian Lindberg. Unless your soloist is a monster like that, even 25 minutes is probably too long. Think of the piece Visions of Light -- it's about 30 minutes long and the soloist gets almost no rest. By the end you're like, okay, I got it. Almost no one can play it, and it might be too much of a good thing.

30 minute concertos can be great, but don't forget that some of the best moments in the best concertos happen when the soloist isn't playing anything at all.

Also the reality is that unless we're talking specialized ensemble, new music gets little to no rehearsal time. Concertos also typically get little rehearsal time. So the longer the work, the less properly rehearsed.

20 minutes is a long time for civilians to listen to a trombone.