Validity of train tickets in Germany

For German regional/local tickets, there are two cases:

• Valid after stamped. You have to start the journey immediately after stamping.

  They don’t always show lines/sections/arrows to indicate this, so you have to read it. The ticket should also say something like "Nur gültig ab Entwertung". An indicator can be that the date and time (when the ticket was bought/printed) is not clearly visible.

• Valid after printed. You have to start the journey immediately.

  They typically show the date and time of the print in a clearly visible way. They typically say something like "Ticket bereits entwertet".

(For longer train journeys, there are tickets that can only be stamped by the train staff. These tickets should show the date range in which they can be used once, and the planned train connection you’re supposed to use.)

How long the tickets are valid before stamping (i.e., when buying them far in advance) might depend on the terms of service of the responsible Verkehrsverbund. The terms I know allow you to use the ticket as long as the price is still the same, and if the price changes (only at the beginning of a year), you can use tickets with the old price until the end of March of that year. After that, you can return them and use their value as credit for buying a ticket with the new price.

How long the tickets are valid after stamping/printing depends on the ticket category. They are either valid for a certain low number of stops (Kurzstrecke) or inside a certain region (Preisstufe, Zone), and there is generally also a time limit, typically depending on the category (I’m not sure if this time limit is always printed on the ticket, but it should at least be stated on the website or in their terms of service). You should have no problem reaching your destination in this time limit as long as you don’t interrupt the journey.

The one-way tickets are only valid for one direction (no round trip), of course, and detours are typically not allowed. In this direction, you can use any kind of public transport which is organized in a Verkehrsverbund (train, bus, tram) and which has local stops, i.e., everything that’s not a long distance train (IC, ICE) and not an intercity bus. If you stay in the time limit, you may interrupt the journey.

The specific one-way ticket from the RMV your question is about, has to be purchased directly before the start of the journey and is only valid until you reach your destination without any breaks. This means you are not allowed to purchase them in advance and use them later.

There are no stamps or marks on the ticket to prove that it was already used. However the validity of the ticket is determined by the date and time printed on the ticket. Even though I can't give you an exact number of hours, this ticket counts "automatically" as invalid if the time of travel and time of purchase differs too much from the time it would have taken you to reach your destination if you started your journey right after the purchase.

According to the transport regulations it is also not allowed to hand the ticket to another person after you reached your destination. If you travel with such a one-way ticket from another person you do it at your own risk of having to pay an increased transport charge of currently 60 EUR.

On the ticket the destination tariff area is printed, probably in your case for Frankfurt (Main) it is 5000. This area basically covers the city of Frankfurt and you can also use the trams to reach your destination within this area. For a detailed plan of the tariff area you can download a map.

The other answers (+1) pointed me towards the answer to your questions but I have the feeling they did not fully articulate what this answer is.

The key is that, in many cases, tickets have to used immediately. That's why they don't need an explicit duration of validity like tickets have in many other places. They are valid for the duration of the next connection between your point of origin and your destination (in practice possibly with some leeway regarding which exact train or connection you are using?)

Train guards therefore do not need to determine whether the ticket has already been used as the opportunities for fraud are very limited. Even allowing for a comfortable time buffer or allowing you to take the second or third available connection, it would be very difficult to squeeze an additional return trip or meaningful intermediate stop.

A side effect is that it is not possible to buy a one-way ticket in advance, as explained in the link provided by @tallistroan.