There's not much you can do but wait and see if he gets back to you on that. You should keep your options open for the time being incase he does end up being a time waster.
Anyways to answer your question there's no set time. I've met people online and seen them in person months or years later (these were not love interests mostly). Me and the last guy I met online met within a few weeks, I wanted to hold off for a while cos I thought it was too soon, but decided to go in the end. I think it's better to get things over with so it's not dragging on for ages, at least you can see if there's an attraction early on. I know you are worried about there not being chemistry over time, but if you've never met and it fades out then no big deal. It may be that it wasn't there anyway.