For anything more complex than a simple sub 5 minute piano piece (which can sometimes come complete from start to finish in one inspirational session of a few hours - although some I have worked on for a couple of months), I will typically take about 6-24 months to write a 10 minute movement for ensemble or orchestra.
I know that sounds like a long time, but I have very little time for the actual writing it down, most of the 'composing' is done in spare 10 minute sections in my head waiting for a train, although I often have to wait a long time for the inspiration to hit. And even when it does, when I finally get round to putting it on paper, I often discard it; I probably reject about 90% of the ideas I come up with (ideas based on a current idea, rather than whole new ideas, if you see what I mean).
So I write very slowly indeed. In about 20 years of composing I have 'written' about 30 short piano works, two string quartets, one 'symphony', 3 or so small ensemble works, a few duets for varied instruments, and about 5 smaller orchestral works. Although several of these still exist only in part at least in my head (some for over 10 years...), as I find the time and energy to commit them to paper.