From what I can tell either until the group she was with despawns or she will last until you RTB and end the patrol.

thank you for that quick response... I was tracking a convoy who I had put 2 fish into each of the 3 merchants, 1 of which sunk another was moving about 2-3 knots and the 3rd I hadn't calculated a speed for because they were weaving and I really don't know how to figure that out yet. What with dodging the escort and all. Anyways my game crashed without a save so oh well. I saved after I set up my boat for the strike so all I have to do is get through the escort again and target/shoot my fish but still, sucks to waste the time for a crash