Startup "Co-Founder" Equity

I'm considering becoming an equity partner in an existing company. I've worked a year with this company, probably 5-10 hours a week. I really believe in the company and think it can be something cool. I've not taken any payment or salary yet.

This SAAS company is making about $5k per month revenue, $3500 profit... so presumably without much effort we'll be at a similar place next year without too much work, if at all. The original founder started the company, quit his job 2 years ago, put $20k into the company, and has been working 60 hours weeks since then. He has yet to take payment or salary for his efforts. He is 55% equity owner. There are two silent partners at 4% each.

I'm being offered 7.5% equity, out of the remaining 37%. I'm expected to work around 20 hours/week for this level of equity. I'm having a hard time evaluating this number. Is it high, low, or what? Just don't have anything to base it own. Any advice?