This is totally discretionary. If you are the true Founder, you decide & if they choose to walk then that's their choice. If this person is absolutely essential and you can't do it with anyone else, best to have the conversation sooner. A good rule of thumb I was told before, you have look at what's in it for you - not what's in it for the other guy. (this holds true for raising capital, as well) Always make sure you're in favor.
Equity is typically vested over a period of time - to make sure that they're going to hold up their end of the bargain vs sign an equity contract and run. What are the milestones you need to hit for this is to be a revenue/profit-generating company? Base the vested equity on this timeline.