I'm getting ready to do a gig on Saturday night, but I'll be singing with 3 others. I personally think that anything beyond 30/45 minutes in a set is too long. A 15 minute break is usually appropriate. Since it sounds like you will be in a coffee house, I'd go the extra time (45 minutes), then take a 15 minute break. If you feel uncomfortable with this, certainly you could always adjust. I think a lot depends on your audience. I have seen times when people were listening so intently and having such a good time, that we just skipped the break. It's a little different when you are by yourself - you have to have your radar on and read the audience. You'll know what to do. Break a leg - hope you have a great gig!

Jim