What equity should a marketing co-founder get?

Short answer

If you join before there is revenue and you are doing the work rather than advising on it, meaningful co-founder equity is normal — often in the 20 to 50 percent range depending on when you joined and what the developer had already built. If you join a product with traction, you are closer to an early employee or a freelancer with upside, and the number drops accordingly. Whatever you agree, vest it over time and write it down before you start.