Ask HN: What's a reasonable amount of equity for the CTO for a small venture?

A friend is in the process of starting up a small commerce venture - focusing on something like clothes (the exact nature of the business is, I think, not so important for this question). He has put a lot of work and effort into design, sourcing suppliers, logistics, legal and IP issues, etc. He's asked me to come on board as a part time 'CTO', responsible for building out the tech (mainly e-commerce website, supplier integration, and company infrastructure (eg mail server, etc).