As the other commenter says, a retainer agreement for a "block" of time is common, but again for simplicity sake I just do it on the fly and we keep each other

As the other commenter says, a retainer agreement for a "block" of time is common, but again for simplicity sake I just do it on the fly and we keep each other in the loop. They keep me informed of current/future things that are coming up and I keep them informed of sizing estimates (e.g. "thats simple", "thats pretty involved", "we should hold off on that and have a discussion to see if we can solve it with a hack then reevaluate")