If you ask any good songwriters how they write their music and how long it takes, you won't find many of them that will just say "I knew exactly what I was gonna write even before I sat down and picked up my guitar. From there it was just a matter of minutes." I would find it very hard to belive if someone told me that, regardless of who it was.
I sat down once and wrote a song in 20 minutes, and you know what... It was cr@p! 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 untill 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 2ns 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 untill 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.