Depends on the situation, clear skys and calm seas 8 to 10 miles, night or bad weather you can be closer.
The main point is you must be ahead of the target and not too far off it's plotted course. Plan an intercept point about 2000 yards off the course and then let them come to you. You can just set submerged at slow speed on silent running and maneuver into a good attack position.
You could
not be in a worse position than what is in the so called Training mission. The only way to catch that convoy is to go submerged at 7 or so knots and you risk detection or just take on the DDs to start with. Not exactly convoy attack training.
If I was in that position I would turn West at flank speed submerged if necessary and try to set up in a better spot. Track the convoy with radar once you get 8 or 10 miles away and can surface. The convoy turns west anyway.
That is why I suggested a War Patrol. Such as Hellcat or even Lifeguard.
In Lifeguard you have 5 big fat, unescorted, CVs to practice on. See the thread on Lifeguard.
http://www.subsim.com/radioroom/showthread.php?t=159594
Magic