I'd approach it like this... Value your time at some number, say $50/hr (USD). Figure 50 hour work weeks, so you should "earn" $2500/wk. Accumulate your time

I'd approach it like this... Value your time at some number, say $50/hr (USD). Figure 50 hour work weeks, so you should "earn" $2500/wk. Accumulate your time until the product ships for revenue. At that time I'm going to presume your time involvement drops significantly and the company starts paying you. You take 75% of the revenues (divided amongst all developers on this plan) until you are "paid off" (or perhaps until you hit 80% payoff). At that time you get 7-10% revenue as "royalties" ongoing. Any additional development work beyond a couple of token hours/week at first for minor fixes should be paid as worked if the revenues support it. You may have to mess about with the numbers a bit, but this approach would give you the greatest potential to get paid appropriately, and still participate in the ongoing sales/success of the product.