If the company already has about 10-15 employees, you're not looking at much equity left, assuming that the other employees get the same you do. If 2 founders and 10 employees got equal amounts, you would be looking at 8.3% per person, and if there were 17 people total (15 + 2 founders, not including you) that puts the company at 5.9% per person.
This is just a starting point, You have to assume that the founders have the lion's share of the equity, and that the amount of equity per additional employee trends downward the later the employee joined. It could be that the company only has 5% equity left for future employees, and so they have to offer only a small percentage of the equity. In any case, try to see what they have available, and again, offer to take only a token salary if anything in return for a greater equity share.
Good luck!