Every now and then I've experimented with a different system, but I've never found anything that worked better.
In the last few days I've tried something which might, just might, replace the thousand a day rule.
It's 500 words in an hour.
The problem with 1,000 words in a day is it's so very achievable. I fiddle around, doing research, writing a few neat phrases, thinking I can finish the thousand any time I like, until ten minutes to midnight, at which point I decide maybe I should get this done, and start writing properly. I finish by 1am, or 2am. That wouldn't be so bad if we didn't have children who needed to be taken to school.
So I've tried 500 words in an hour, and so far I've impressed myself. Productivity has gone up. (But don't tell Janet, Kathleen, Keith or Belinda, or they might start having unreasonable expectations about me getting stuff done on time...)
With the 500 word per hour system, when the hour starts I don't allow distractions and I hit the target. Every time. Amazing how much those distractions hurt.
Word count for a book varies wildly between genres. Writing historical mysteries, I've always targeted between 80K and 100K, and no one's ever told me I should do otherwise. Beyond 100K, my observation is agents and editors become skittish. Below 80K, I become skittish. Anything within that range is goodness as far as I can see. Once I'm in the zone, I don't care how long it ends up.
In theory this means I finish a first draft in 80 to 100 days. Yeah, right. Still, I'm probably not too far off that. That's not the end of the story though. Personally, if you're using a target system like this, I suggest allocating three times as long for revision as you spend writing the first draft. Yes, three times as long. I'm sure there are people out there who can get it right the first time. In fact, I know there are. But I'm not one of them. On the plus side, when I send in an ms, I know it's as good as I can make it.