Hee-hee, sorry, couldn't resist. There is an option in the realism settings to turn off realistic sinking times, but even with that it sometimes takes 10 minutes or so, depending on the flooding rate. I generally wait at least 10 minutes if I have the time.
Two fish in this carrier at 19:49, three minutes later (lower right, 19:52) it's listing about 10 degrees. After 10 minutes the list hadn't increased, and it hadn't settled any lower in the water, so I sent one more fish into her.
Two minutes after the third fish hit, edge of flight deck is in the water, looks like she's a goner.
Yup, three minutes after the third hit she rolls over.
When enough is enough is a judgement call, a lot depends on how many torpedos you have available, how important the target is, and if there are three destroyers bearing down on you or not! Merchant ships tend to be easier to sink since they have less watertight compartments, and just as in real life I've found that rough weather tends to swamp or capsize them quicker once they're damaged.
"effective tactics for sinking destroyers?"
Fire everything you got, then quickly put in for a transfer to shore duty?
I try to avoid anything designed for the purpose of destroying a sub, for obvious reasons. There was one US sub skipper, Sam Dealey of the USS Harder who deliberately targeted Japanese destroyers, but only after getting intel that the Japanese were running short of them. In actual practice thru most of the war a destroyer or escort was considered not worth the risk, since a sub and crew cost a lot more than a tin can. I don't know about this game, possibly you get more renown for sinking a bunch of destroyers rather than the equivalent tonnage of merchants, I haven't experimented with it yet.
The difficulty has a lot to do with that, hitting a destroyer moving at high speed and making frequent turns would be like trying to line up a bow & arrow shot at an energetic fly. Best tactic if you gotta is probably the same for any sure shot - get as close as possible on the beam, aim for dead center to allow for turning and speed changes, fire a spread.
"One more: what is the absolute minimum range for the torpedoes?"
You know, I tried to find that in the manuals, it doesn't say. I have heard 300 to 400 meters, altho I tend to fire somewhere between 500 and 800 meters.