This one's hard to just give a blanket answer to.
You want the person to be committed to the success of the company for the long term, so obviously, you need to give enough equity that he cares.
At the same time, you need to be fair to yourself, keeping in mind that you will be diluted in the future, and the math needs to make sense for you eventually.
I'd say somewhere between 5 - 10%, but I'd probably try to trade some salary for equity. Not with the intention of being unfair, but to align interests.
Market salary deserves market equity, which would be around 1-2% for a company at seed stage. But, for a senior position, that low amount for equity does not bode well for the long term.