For me it all depends on how quickly the ideas flow. Sometimes I can pretty much write a whole piece in one long night, even for a decent-sized instrumentation, or I recently rattled off a 4 minute piece for solo recorder in one evening, but other things take me ages. I finished my three pieces for brass choir which vary somewhere between 3 and 7 minutes I think. First two I each finished in one month each, the last and longest one which was based on a text took two months. An 8 minute set of variations for horn and orchestra took a single month. Now I'm struggling on some piano solos, tuba sonata, and other stuff for a couple months now. Every once in a while I go back an add a couple measures to my brass quintet, but I just can't sit there and work on it, I get stuck.
So really, I work two ways. Either all in one go, which might take my anywhere between 1 evening and 2 months, or little by little just taking pecks at a piece when I feel like it, which my take... a year.

I struggled with 2 movements of my bassoon duet in maybe two months or so, then hadn't touched it for 8 months or so, came back, and wrote two more even longer movements in one month. So go figure.