My rule of thumb - about 10 - 12 songs + extras. Works out about 5 mins per song including getting started and finished + applause + introductions. If you need extra - slot in before the final song to get your big finish. Intros are good for spacing out as an hour of non stop singing can take a toll on the voice. Also make sure that the intros are interesting (to the audience), sufficiently rehearsed (enough to know what you are going to say before you say it) and don't ramble on too long.

Also, when you are rehearsing, keep note of timings - to get a balance of longer songs and shorter songs.