There isn't really a standard. If you know what the advisor would normally bill for their time, I would start there. Determine how many hours they will put in and then what they would normally bill per hour. The next part is the hard part. What is the startup worth? We sometimes will wait until the first round of funding and extrapolate out the stake then based on the valuation, but we also normally insist on part of the payment in cash. You can also set and agree to a valuation at the time the work starts, but that could bog you down in a lot of negotiation.

Both sides need to agree on deliverables and value of the advisor to the startup getting off the ground.

Sometimes an advisor is so impactful, that it might make sense to offer closer to founders level shares, but that is all in the determination of value.