I personally gave 5% to a trusted advisor over a 1-year vesting period, but that was based on a very low valuation and for a (still) bootstrapped startup. I think we're sort of agreed on a target valuation of 1M$ so my advisor basically agreed to make $50K out of it. He told me he's been counting his hours, but lately he has worked way more than his $200 consulting rate (so in retrospect I realize he measured his expected ROI in terms of time spent advising me).
Note that originally he only asked for 0.5%-1% but I made him realize my company was a lot smaller than the startups he typically works with (and have already raised some funds).
To be 100% fair and straightforward, I would advise bluntly asking how much the advisor would like to make out of his commitment to you and then devise a good percentage based on a target exit valuation (exit for the advisor, not necessarily you, of course).