Depends on the Advisor and what you use them for. Are they making intros? Coaching your leadership team? Providing product, market or management insights? Each requires a different level of engagement, but I would definitely clarify and set those expectations - even if you didn't do it up-front, do it now.
In many advisory and mentor situations I've not really known exactly what the company wants and I'm sometimes not sure I'll show up in the right way or at the wrong times or communicate through the wrong channels, etc. - so everyone will enjoy clearing this up (oh, and update this clarity as your situation and needs change). To set these expectations, have a discussion. Probably start by asking what an advisor can commit to or what they prefer. It should be a discussion / negotiation rather than a demand, even if you're compensating an advisor with equity.
I am a formal advisor for several startups and I find the engagement is usually more organic than pre-scheduled dates/times, mostly because schedules don't allow it. We set a rough expectation that we will be sure to connect every x number of weeks, then each of us is mutually on the hook to reach out. Usually they reach out when they have a specific need and I reach out when I plan to be nearby.
In my experience, Advisors are compensated with equity. Often, there is some expectation of commitment included in the advisors agreement, and either party can terminate the agreement at any time.
I serve as a formal advisor for several startups.