My band has played a few times, but only at talent shows, so we've only been able to play 1 song at a time. I was wondering how many songs it takes to fill up an hour set, or 20min or something like that. Is there some sort of general rule, like 5 minutes for every song? Thanks.
Make sure yours songs are the ones you like the most. Don't pay attention to the length, just add some songs you really like up to meet the (about) length-requirements and theme of the show you're performing at. In my opinion, artists should play whatever they want as long as they don't go over their set and they keep the crowd entertained, no?
But in the world of sharing stages, following set times, and just general plain old 'doing business' there are norms that are typically followed.

Generally an opening set is about 45 minutes or 10 songs. (which will be about 45 minutes for most purposes). It is generally considered impolite to exceed your welcome. If you're asked to play 'one set' and get up and play 10 tunes at 12 minutes each, you will NOT get called back.

Generally, a headlining set is between 60-90 minutes, maybe more, but probably closer to 90. Essentially two sets in one go. 20-ish songs. Unless you have more than one opening act, in which case, the expectation will be that your set will be more like 60 minutes.

As a cover band, you're looking at doing 45 minute sets, with a 20 minute break in between each set. You'll probably do 3-4 sets, depending on the venue and audience.

For festivals and such, you're typically looking at 30-45 minute sets. Sometimes there will be pressure to squeeze two bands into a single hour slot, meaning a significant amount of gear-sharing. Otherwise, the stage will be booked in one hour intervals, so courtesy says to keep your set to a max of 45 minutes, allowing for a 15min changeover. Get yer gear off fast!! Politeness and courtesy!

Notice the pretty consistent focus of 45 minutes?

If you are asked to play an opening set and get up and do three songs and you're off in 15 minutes, most people (yes, even the audience) will be surprised that you bothered and will wonder why you stopped so soon. (unless you suck, at which point, they will be grateful....but that's why most clubs and what-not won't book you without hearing you first).

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
thanks for that. I just didn't really know how many songs would take up 45 minutes, because there is time in between each song, and introductions etc. -mnin
axemanchris is dead right about all that stuff. another tip, pirticulary if your doing a 45 minute set is tie songs together. go on stage, make your first two songs your better ones, and dont stop between them. introduce your band after that. it looks more professional than if you play each song and talk between every one of them to tell them what the song is and stuff.
General rule of thumb, allow 5 minutes per song.
Some are over 5 minutes, some are under 5 minutes, but it generaly evens out to 5 minutes, also this allows a minute or so talking time here and there throughout the set.
So 12 songs, with a bit of talking in between, would be around an hour of a set.
Of course, this all depends on what sort of music you play, if you play short, fast punk songs, one after another like the Ramones, you can probably nearly double that number of songs, or if you play in a progressive rock band who's songs last 15 minutes each, you're only gonna fit four songs into an hour set.
But for me, the 5 minutes rule works fine. I can count out 12 songs, play the gig and finish, almost exactly to the minute in some cases, approximately one hour later.