Hi there,
Yes, congrats on the potential client!

I bill in 15-min increments using hourly billing. If you only deal with one email every few hours or more, I would bill 15mins for ea instance. If you're dealing with a constant stream, then it would probably be just however long it took overall @ your hourly rate.
It's more cost effective for someone to have me work on a large chunk of tasks at a time rather than send me blips of things throughout the day... but that's just how I've always billed. I know there are big opinions on this!
HTH & hope it works out!