5-10% depending on the quality of the developer is what I have done in the past and as soon as we start making cash they start getting paid. Also I only do this deal with folks I know well as it is so much more complicated than standard hiring.
Unless there is a surefire payoff, get some investor money and pay them what they are worth.
If you know the person well you still need to be careful with the contract but you should be less likely to fire them and get into an argument about equity.