The bonus is for a specified amount of paid rake, if you have Poker Tracker you can see how much rake you paid for a specified amount of hands. The sign-up at VC depends on points, so if you play say Heads-Up or Fullring the amount of hands will vary greatly as the size of the pot determines how many points you get. I usually check the details of the bonus from the affiliate / poker site and then use Bonuswhores charts to make an educated guess of how many hands I will need.
Bobo, what was wrong with my link? I haven't heard that it would be unacceptable to post links to an affiliate, there was no referral link