In Frankfurt the RMV does not use ticket validation machines, therefore, the tickets are timed. IIRC, the tickets are good for a fixed period of time after issuance (maybe 90 or 120 minutes, but I'm not sure).
Again IIRC, the 4-zone single ticket from the airport to the Hbf is €3,25. Since the 4-zome unlimited day ticket (Tageskarte) is €7,20, it may be a better bargain if you are going to ride throughout the day and evening.
And the ticket-checking police DO check for valid tickets - the only two cities I've ever been spot-checked in have been FRA and Berlin.