I've heard that it's around 2%. but my take is this... put in a vesting agreement where they need to deliver before getting whatever equity.
Whatever you do, make sure that you map out what are some of the things you would want that advisor to contribute. Advisors who really care won't mind the vesting period.
Vesting period is really a period for both sides to deliver what they say they will do. If the advisor does his role as a good advisor, he'll be rewarded for it. If he doesn't, you are protected.