http://news.bbc.co.uk/2/hi/technology/4900742.stm
Personally I stop pretty much before page 3 depending on how the results have been for the first two pages…
Usually after the first page I rephrase the Search if it doesn’t appear to be going where I want it to be. But if I’m getting the results I want about page 4.
I find the search normally goes off topic after the first couple of pages anyway. So like Drezha I normally rephrase or try a different engine.
in health care etc, so many words or phrases have a sexual connotation that we find we are getting x-rated sites by halfway down page 2 anyway. so i rarely go past page 1.
page 1 here too 
Normally about page 10 I call it quits
Think 31 was the largest or there abouts i’ve checked though to.
i normally reword what im looking for