When to give up surface attacks

Ahoi!
So my question would be what is the date when it's time to cease conducting night surface attacks against convoys due to enemy radar activity. In my current career it's October 1941 and escorts are almost always detecting my presence at night even in stormy weather so I really mean abandoning the surface runs. I wonder when was the latest occasion in the war when you did manage to make a classic early-war surface attack on an escorted convoy without getting detected by radar.
Thanks for your answers!