Depends how 'complete' it is. How much care someone has taken to edit their own work first. Heimlich's SM IS the longest SM out there, and yet it needed very little editing because Heimlich had made sure he'd gone over it thoroughly first. (probably second, third and fourth too) He'd had other players play it too. So they'd picked up things as well.
I've taken the longest over the ones that you know will be 'OK', but need so much work you don't know where to start. (OK shadowchi excepted, where that just dragged on because I kept getting side-tracked with other projects). Like do you attack the writing in different tenses, or do you attack the mechanical side of things where authors haven't made it so the evidence once linked to a suspect is revealed on the case page. :s or there just aren't motives any displaying.
It also, to be completely honest, is a case of 'interest' for me. Some stories interest you so you like to edit them; they're a pleasure to read. I dropped what I was doing to edit Heimlich's coz it was a good read, and worth my time. And some, despite how well written they are, well they just don't interest me. I'd sooner read bread. SO I drag my feet on those.
AND lastly, it depends on how much spare time I have. What am I doing on Noir / Shades, and occasionally I like to have a life. For me, real life will always take priority over Cyber-life. :)
I think that's a good answer 'for me'. Others opinions will vary.