Actually, the milestone you have to meet every 12-18 months, in 500's case, is raising a new fund. Given their speed of investment and quantity, this is an incredibly herculean task. Definitely not easy, and very easy to fail - the failure mode here, of course, being not raising another fund. Sure, the first fund might do well, but 10 years later 500 will have failed as a VC if it has only raised one fund.