What equity should I expect in this scenario?

I'm interested in joining an early stage startup and have a question regarding what is the expected equity I should be offered. This is the scenario:

1. The startup has a single founder who is the only person working on it. It has a working MVP but has some issues that are limiting it's functionality.

2. The startup has some users but nothing super remarkable yet in terms of traction or product/market fit.

3. The startup is currently in an incubator, but hasn't received seed money. The incubator + mentors have 10% equity.

4. The founder is looking for a CTO that can build a v2 (for an upcoming demo day in 3 to 4 months) and in general take over the tech side.

5. The existing founder is working full time, while at the moment I'm available to work on this part time only.

6. The existing founder has paid the startup's expenses up to now.

After talking to the founder, I'm being offered "5% to 15%" after vesting, with no salary, for a part time role as CTO with the main goal of getting v2 up and running for demo day.

I don't mind the no salary part since I'm also interested in participating in a startup and the learning this can bring, but it seems to me that 5%-15% is low for such an early stage.

Is this percentage normal? If not, what would be an expected percentage?

Thanks