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.