It depends on a lot of parameters.

a. Motivation: is your dream to dump your day job and join a startup? If not, stop.

b. Savings: can you afford to make almost no income for a year or more, do you have student loans, unpaid credit cards (ignore mortgage debt, it's usually healthy). Is there a minimum income you absolutely need monthly?

Then there is the actual job. Will you be the first dev for the product, do you consider the job a quasi-founder position or just dev #n?

Once you figure all of this out, the right deal can be anywhere from:

Full co-founder: no salary, 20% - 50% equity (with vesting of course).

First dev: 5% - 20% equity, with a third to half of your old salary

Dev #n: 1% - 10% equity, half to 2/3 of your old salary