Hi
I often have times like many of the people on here - short bursts of quick working and longer periods of frustration. I think there are so many parameters it is hard to say - pressure to complete something for someone else, or writing a hobby piece to make yourself proud will make a huge difference.
In general I write fast, but my stuff is often more about ideas getting thrown out there , than well-thought out developments and structures...although as I'm getting older these latter things start to appeal more and more.
I also find that my emotional attachment to pieces I'm writing only lasts so long, and then I lose my momentum. Unless I working for someone else, I find y own drive goes on a work after a few weeks - but having said that I tend to write smallish pieces, as I simply don't have time to write on 80 stave paper...much as I'd like to!