You will get 100 different answers depending on who you talk to, but it is always a combination of many factors:

1) avg pagerank of top 10 should be low (lower than 2 maybe)
2) are they just the tld, or a deep page? the more tlds, the tougher
3) are the 4 blocks to the right all Yes? if not, then they might not be trying to optimize that keyword
4) Can you get more blp's than the sites listed?
5) are all the domains aged like 10 years?

Every keyword is different. there is no magic bullet numbers. In theory, the more red on the page, the harder it will be.