[UKönig]

I only use time compression to travel long distances, or sometimes to help just get me in range a little faster. Once the action begins TC is used to speed up the torpedo until it gets close enough to strike, then I slow it down to watch the ship get hit.
If I am the hunted, I almost never use TC, I do it 1:1 until they (hopefully) lose interest and go away.
A lot of real time can tick off the clock while you're evading an attack.
I played the in-game single mission "break through to gibraltar" twice, with different boats. The first was the type VII they give you (U-96 from das boot) and the second was a "let's try 'what if' " and I used the type XXI for the exact mission. In the type VII, I started it at 4pm (my time, real time), and by just after Midnight, (my time, real time) we pulled into La Spezia. I was so intent on avoiding the english fleet that 8 actual hours went by without my noticing.
That same trip in the Type XXI took 2 actual hours.
Above all, never use TC when being depth charged, too much happens too fast for you to react and it is almost certainly 'fatal'...