I sat down once and wrote a song in 20 minutes, and you know what... It was crap! I've come to learn that if you just settle for whatever you can come up with in less than an hour, it usually won't be a great song.
I mean think about it, not only do you have to come up with the words for the song, you have to write the music too. For some people that may be a little more talented than others, writing the music could take 30 min. Or 3 days, or even 3 weeks. Whenever I sit down to write a song, I just go to a quiet spot in the house and just start playing, I don't think about what time it is, or what else is going on, I just think about the song. I'll play around until I finally get an intro to the song. In the intro, it's up to you whether you write chords or a certain lead riff. From there the music can pretty much write itself as long as you just take your time with it and try not to really think about it too much. Once you get the music written, practice a couple of times so that it sticks in your head and you get a real good feel for the song.
Now on to writing the lyrics. What I normally do, and this works (for me at least), is just play the music and sing whatever comes out of my head. If you're not satisfied with it, or you just don't think it fits the song, then start over and try it again. once you get a start to the song the verse is done before you know it.
This is when you actually have to think about what the entire song is going to be about or what it's going to revolve around. just do the same thing that I said in the previous paragraph and start writing the 2nd verse or the chorus, whichever comes next in the song.
From here, it's smooth sailing. Just take your time with the song, if you get stuck just play what you have up until the part where you're stuck and keep trying to add something onto it everytime. You'll eventually have something finished and you can just take it to the band after you're done and mold it from there.