Greater risk = greater equity. How likely is this to fail or just break even? If you aren't receiving salary yet are among 4-6 non-founders with equivalent sweat investment, all of whom are lower on the totem pole than the two founders, figure out:
1) Taking into account all likely outcomes, what is the most likely outcome in terms of exit? (ex: $10MM.) Keep in mind that 90%+ of all tech startups fail (Allmand Law study), and of those that succeed 88% of M&A deals are under $100MM. Startups that exit at $1B+ are so rare they are called "unicorns"... so don't count on that, no matter how exciting it feels right now.
2) Figure out what 1% equity would give you in terms of payout for the most likely exit. For example, a $10MM exit would give you $100k for every 1% you own.
3) Decide what the chance is that the startup will fail / go bankrupt / get stuck at a $1MM business with no exit in sight. (According to Allman Law's study, 10% stay in business - and far fewer than that actually exit).
4) Multiply the % chance of success by the likely outcome if successful. Now each 1% of equity is worth $10k. You could get lucky and have it be worth millions, or it could be worth nothing. (With the hypothetical numbers I'm giving here, including the odds, you are working for $10k per 1% equity received if the most likely exit is $10MM and the % chance of failure is 90%.)
5) Come up with a vesting path. Commit to one year, get X equity at the end. If you were salaried, the path would be more like 4 years, but since it's free you deserve instant equity as long as you follow through for a reasonable period of time.
6) Assuming you get agreement in writing from the founders, what amount of $ would you take in exchange for 12 months of free work? Now multiply that by 2 to factor in the fact that the payout would be far down the road, and that there is risk.
7) What percentage share of equity would you need in order to equal that payout on exit?
8) Multiply that number by 2-3x to account for likely dilution over time.
9) If the founders aren't willing to give you that much equity in writing, then it's time to move on! If they are, then decide whether you're willing to take the risk in exchange for potentially big rewards (and of course, potentially empty pockets).

It's a fascinating topic with a lot of speculation involved, so if you want to discuss in depth, set up a call with me on Clarity. Hope that helps!