I received my SGS2 yesterday and, as I have not had the chance to transfer all my data over as yet, I have left the phone on idle since it's first charge.
As I am sure most people will have started using the phone immediately after they received it, I thought some idle battery usage data would be quite rare and potentially interesting to potential buyers.
Here are my results (hopefully you should be able to make out the rough times on the following picture as verification)
100% - Tuesday 3rd -10:43pm
81% - Wednesday 4th - 5:18pm
The total time the phone has been off charge is therefore 18 hours 35 mins, in which time it has lost 19% battery.
A few calculations:
1% battery loss per 58 minutes
100% to 0% - 97 hours 48 mins (4 days 1 hour 48 mins)
This seems to be quite a lot better than my SGS 1 running Darky 9.2. I have not done a similar test, but I would suspect it would not last much more than 2 1/2 days.