From my (somewhat limited, but concrete) experience, there is no way to get a satisfactory answer to this question. Just value the options at $0 and set your salary expectations accordingly.
My answer was perhaps a little tongue-in-cheek, because I agree they won't. You'll hear things like "we want to encourage our employees to give it their all" and "we believe performers should be well-compensated," but those don't really answer the question.
You also might be told "you are in the same pot as the founders," which is technically true except that the founders can still be never-work-again-rich after a 90% dilution, and the founders will typically have a seat at the table when the question of "how do we re-up people who have been diluted so low but are still capable of spiking the deal if they don't like it?" comes up.