How much equity I should ask for as the first employee of a startup with 2 co-founders?

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…