Best timing for snorkel?

[Shkval]

Originally Posted by Hinrich Schwab

My routine is battery only during the day. After dusk, snorkel to full charge and alternate battery and snorkeling every three hours. Top off the battery around dawn and repeat.

At what speed during the day... and night... 2 knots...3? And how long is the "black" period of time when Brits got radar and "we" don't have snorkel or any kind of RW... 6 months or longer? Was that the "Black March"? I remember the first time I got hit in the middle of the night out of nowhere... very unpleasant experience...
What do you do in those circumstances?