Weird question, i know, but...
 

...what would be the average time for the crew of a merchant ship to get into their lifeboats after they've been warned by the U-boat that the ship is going to be sunk? I know the game doesn't model this, but I like to roleplay a bit, so in early war, when air cover is not so threatening, I "stop" the merchant with deck gun shot across the bow and then give them a bit of time to save themselves. So, how much time would they need in real life?

In a calm sea It takes about five minutes to swing out the lifeboats and lower them. You would need to give time for the crew to assemble, take a few necessities with them. Get the engine room gang out, hold roll call.

Probably 30 minutes would be reasonable.

30 minutes is a good time, but imposible to roleplay in the game. After 30 minutes ship will be 5-6km away from warning position. I think 5-8 minutes in game is enought.

A matter of a few minutes if rafts were employed and depending on the level of urgency but I'd allow 10 minutes in game time.