I am sure I have come across the answer but I haven't been able to find it again. My not very good memory thinks about an hour.
I would also like a reference if anyone has one.
The wiki for Becuna says (Submerged: 15.0 knots (27.8 km/h) for ?1?2 hour), but I think that is for a Guppy configuration with Sargo II batteries.
Thanx,
Drew