It really varies, sometimes they flow out in a continuous stream in a couple of minutes, then I sit down with the words and a guitar and allow the melody to develop, and put chords to it (I'm not good at bar chords so they tend to be fairly simple chords) Sometimes though, like with my first song, I wrote it as almost a 12bar blues, then about two years later I started learning bass and the bass line I came up with was begging to be with those words, then a friend suggested a couple of chords, and there it was, my first presentable song !
Usually it's a fairly quick process though, strike while the iron's hot, and I don't try to write them if I'm really not feeling it, it just doesn't work
