A thread at Cre8asite Forums named Ranking Check Depth discusses the topic of how deep should one go when checking their rankings. Many people I know check their rankings either with a tool or manually. Either way, how deep do most people go when they check their rankings and how deep should they go?
I personally do not go beyond the 100th result, or 10th page of the search engine results. Honestly, anything beyond the 5th page, might not be worth checking. But a 100 results is a good number because (1) most engines let you show 100 results on a single page and (2) most search engines provide 1 - 10 links to the results pages.
Check out the thread to see what others do and why they do it.