score:4
It is deliberately flexible enough to allow you to travel easily. If something feels reasonable to you in the context of a single journey then it is almost certainly fine.
Could you get away with a more complicated route (using the flexibility as a loophole)? Almost certainly. Unless you are really pushing the boundaries then it is down to what your conscience can justify.
Similar European cities have rules like the last leg must start within one hour of the first stamp. This is probably a reasonable guide. If your journey is much longer than this then you may be doing it wrong.
Upvote:6
Edit on November 7, 2017: Note that Wiener Linien decided to discontinue the 90-minute-ticket which this answer is about. The 90-minute-ticket won't be available anymore on and after January 1st, 2018. End of Edit
This is not a direct answer to the question, but per the request of the OP I'll expand my comment into an answer: Besides the normal single ticket there are also 90 minute tickets. These are only available via the app which is available for Android and iOS. It costs 2,80€ (so it's slightly more expensive than the normal single ticket which costs 2,20€) but allows you to travel freely in whatever direction you want for 90 minutes (so you could even go back and forth).
To buy these tickets you need a mobile device with Android or iOS and data (maybe it's even enough if you're connected to the internet at the time of buying the ticket but the validity of the ticket starts when you buy it). They also ask for an address but you don't need a local one (you should even use your home address as they use it also as the billing address). You can pay with paybox, paypal, Visa, MasterCard, American Express and Diners Club.
Note that for 5,50€ you can already get a day ticket, so if you know beforehand that you want to make several trips in a day, get a day ticket.