Ask HN: What stake should I have in a company when I'm developing for free?

I've been working on a website for some guys I know based on an idea they had. The premise was that I do it for free and I get to own part of the business when it goes live.

Does anyone have any experience with this sort of thing? What % of ownership should I have?

This is not my idea, but I've spent 100's of hours developing their site for them while I've been on break from study.