1. You should probably have the discussion as soon as possible.
2. If you are the only engineer, you are their most important asset. No other department has anything to do until you have a product (except for maybe market research)
3. Ideas are a dime a dozen, implementation/execution (and arguably marketing) is where the battle is won/lost, and you are currently writing their implementation/execution. You are very important, and your equity should reflect that.
If there were 3 people I would expect you to get 33% or more, especially if they don't have the skills to help/contribute.
This is not to say that you're invaluable (they could technically just get another engineer, or try to pick it up themselves as they go), but you should definitely not undersell yourself/settle for some absurdly low amount of equity when you're doing the heavy lifting.