I sometimes do the "write as much as I can in fifteen minutes" strategy. Then I make myself write for fifteen minutes more. Or I decide "this is going to be roughly four pages long in Word" so I have a target length. It really depends on what I'm trying to accomplish. For the last three pieces I wrote, my goal was simply to meet the requests, and I didn't want to write very long fics. Basically, what I try to do is first figure out how long do I want the fic to be, because that gives me a framework on how quickly I need to wrap up the story elements, and then I set aside how long I want to write and what date, roughly, I want to be finished with the story. I tend to view my writing as "if I was writing this for a challenge, when do I need to be done, what needs to be in this story?" If I'm really stuck, I'll ask for a beta just so I have a sense of "is it working so far?" The latter usually helps me not only identify what I need to fix, but also gives me motivation to finish it.

Hope this helps.