Here’s how it’s supposed to work:
If you’re not currently active on the site, notifications get sent via email.
There is a (currently) a 10 minute delay, to allow for ninja-edits to posts before they get copied into notifications.
You can reply to messages directly via email. Incoming email replies are pulled into Topics every 5 minutes.
Worst case, this means a 15 minute round trip if you’re conversing via email. These time limits can be reduced, but it may introduce excess load on the system. Let’s see how we go with the defaults for a bit.