My main point is that you should at least give a rough lower bound for the number of hours you're expecting of commitment per week, because the OP just says "a few hours per week". What's "a few" here if not 3-5? 10? 15? At that point you're expecting about the same commitment as a part-time job.
The project seems to be mostly asynchronous anyway so expecting a general minimum time commitment doesn't seem that helpful. Having 3 hours per week to dedicate to tasks isn't that bad as long as you're communicating outside of that so you know what you're going to work on, which I think most people would be totally fine with.