As I said, each project, each work unit, will have a certain track record of how long it takes in a given environment. For me, the fastest GPU work units take just over an hour; the slowest SMP work units take 3 days; and some recent CPU work units I run on an old uniprocessor incapable of SMP were running over 7 days; the GPU work units on my older AGP bus ATI HD4670 take about 11 hours. And, everything in-between.
I think there are some databases around for expected times to complete the different work units, but there is so much variability to hardware and environment, that you may want to do it yourself as you go along. There are third party tools, e.g., HFM.net, that will keep stats like that for you in the old v6. There has been considerable discussion that when Stanford's new v7 client comes out of beta and has its data structures and APIs stabilized that third party sites will provide such new tools for v7.
If you haven't looked, there is a link at the very top of the forum page called "tools" that takes you to a list of various tools that can help you set up, monitor, evaluate your hardware, etc. None of them yet are specific to v7. While they don't keep track of individual work units, third party stats sites continually analyze a monster flat file that Stanford provides each hour and makes the analysis available to individual users and teams. I frequently look at Kakao and XCPUs web sites - links are in the tools link.
The big caveat seems always to be, "YMMV".
EDIT: here's a snippet of a couple of days of production for my Sandy Bridge system - you can see the wide variety of work units, the "Frame Time", which is the time to complete each 1% frame, and the points/points per day. I still run this one computer in v6 so I can continue to get these stats from the HFM.net tool. I run v7 on my smaller systems. [sarcasm on] I remind myself of the movie "Men who stare at goats" as I ponder these lists...
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[/sarcasm off]
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