The startup was recently launched with very little marketing strategy. It is not funded, and there is no real traction at this point. However, I believe in the vision and I know the domain well. They will also get press coverage in 2-3 weeks.
I was approached with an 'equity-only' offer as a content strategist/marketer for the long term, 20-28 hours per week. I got this offer as the 'first employee'.
Any advice on how much equity I should ask for? I am not sure I can propose to be a 'co-founder' at this stage, but I'm also unsure that 'employee' makes sense.
If they are offering equity as your payment then I'm surprised that they didn't come with a number. Or maybe they will. It's hard to give a firm answer because it all depends on how much money the two founders are investing on their own. If you say you'll invest equal portions then you can ask for 1/3 of the company. If you're just investing time, how much time, compared to how much of their time plus equity. Depending on those answers, the amount of equity you're worth can drop pretty low.
There is no rubric for you to follow. Negotiate with them what you think you are worth as far as value to the growth of the bottom line, make sure to also consider possible upsides for the company you will be joining not being greedy for stock alone, consider shorter vesting period.
How many more employees will be joining, what does the team plan on doing with the rest of the equity... how much funding will it be needing in the near future.
All these can be argued as "not your problem", but when you join should consider things as a team.
Whatever you do, get it in writing before you start investing too much of your time. Depending on your comfort level, if there are only 2 co-founders right now, I would ask why they would consider you an employee and not a co-founder especially, as you noted, they have no marketing strategy and no traction. It's a pretty important conversation to have. The comments by others are also spot on.
Best of luck. Set-up a call with me if you like.