hello, how do i set a swim workout by time and not distance?

Currently the only possibility I see, is using the step "Rest" instead of "Warm up", "Swim", or "Cool down", even if you won't rest. At the step type Rest you can set the duration. You can add explanatory notes to the individual rest steps, so that you know what they were intended for, and whether they mean true rest, or swim in fact.

... however, I do not know how you intend to make a break if the time for the current step runs out just when you are in the middle of the pool. From this point of view, estimating the number of pool lengths you need to swim, and entering the pool laps instead of times, makes much more sense, because in such way it is assured that you can make your breaks always at the pool end.

Thank you very much for your answers  
happens all the time when I didn't finish 25M and the watch buzzes thinking I'm done... same here, it will buzz you when the time you wanted to swim ends, and you will push the bottom when finishing the pool.