The help text is misleading. It's talking about the whole scenario described and not just the time interval filter. As far as I have been able to tell, this is a global interval and is not going to store the interval per device. If you want some help with a script that will do this filtering for you just let me know and I can come up with something like this fairly quickly.
(Vbscript - your preference or mileage may vary)
You just need to check that INStr(VarCleanMessageText,"FAN") <> 0 and INStr(VarCleanMessageText,"ENVIRONMENT") <> 0
Then add the hostname to a dictionary with the current time if it's not already there. If it's there, diff the time and alert or skip to the next rule.
To answer your other question, I don't see any set limitation other than the number of characters you can type into the box, but just for testing purposes, I just setup a rule for 120 minutes and in 2 hours we'll know if it works for sure.