That's a good one! Very hard to answer question from both sides of the deal, the advisor and the company.
To help you decide maybe separate the work to two: general advisory/ business development and VC contacts (in that I guess you mean - "get me VC investments" - right?).
For the general work you need to estimate the amount of work that will be done by the advisor but keep in mind that most companies allocate 5%-10% of their equity for ESOP (Employees and consultants Shares Options Plan) and from what I've seen advisors usually take 1%-2%. However, for the VC contacts (and make sure he/she are not "brokers" if they are not licensed as such)you can offer a percentage (usually between 5% to 10%) from the investment or a percentage from the issued shares in such investment (again 5%-10%) or both.