A possible way to structure that business relationship is you have identifiable projects where Real Work gets done. You do the whole contract / invoice / etc song and dance for those, at project rates.
In between projects, you make it known that you are a friendly guy, and bending your ear for a few minutes is free. N.b. This is how lawyers will generally work. This is to maintain customer loyalty for the five figure engagements. Plus, since you're doing it out of the goodness of your heart, you have an instant, built-in non-excuse for unavailability.
In cases where I've been hired for an identifiable project, that's what I've done. But not all consulting fits that mould; the most interesting I've done has been in the category of "be available for random stuff we throw at you, and send us a bill at the end of each month".
In between projects, you make it known that you are a friendly guy, and bending your ear for a few minutes is free. N.b. This is how lawyers will generally work. This is to maintain customer loyalty for the five figure engagements. Plus, since you're doing it out of the goodness of your heart, you have an instant, built-in non-excuse for unavailability.