Startup Advisor Equity

Founder Institute has handy system for come up with the short answer at this link.
They figure the range from .15% to 1.0% depending on the level of contribution and the stage of development of the startup.

The smart people on Quora.com give a range of .25% to 7.0%. It depends on how much value they bring to your startup.  Search Quora for Advisor Equity.

The Startup Toolkit suggests a normal range of .5% to 2.0%.

These are all guidelines, but of course all of life is a negotiation!