It depends on the weather, the condition of the seas, is it day or night, how close you are to the target. If you play with your noise meter on (sub that changes from green/yellow/red) once you have been red for several seconds, usually you have been spotted. If the watch crew is croached then you have been spotted. (If your surfaced). Also you can be spotted if you keep your periscope up too long and your close enough to a ship.
Hope that helps.
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