See this interesting blog posting about the value of a Consulting CTO. Consulting CTO’s can help you out on a short term basis; generally for small amounts of equity.

Depending on the type and amount of work; you will end up giving 25,000-100,000 stock options (where you have 10 million outstanding @ .01 – .10 cents). These will generally vest based on the terms of engagement and deliverables you require from the freelance / consulting CTO.

My recommendation; as someone who actively plays this role for multiple startups is to be very clear up front on the scope of involvement, deliverables and compensation. Startups are by nature fluid. The core team and all need others involved to be flexible; but putting some structure around this at the beginning can lead to a successful relationship long term.