If you cross Dover Calais it should take 10-11min. You should be very unlucky to lose your engine over this 22 miles of water. I don’t think that any one can tell you how long the a/c will float. I know of someone who ditched an Arrow in the Med. in later years he crashed into water and was killed. The Channel is a very busy shipping route. If you ditch parallel and forward to a ship they should hopefully be able to see you. Also, if you fly high enough you might be able to glide to a point near the coast. Winter/Summer are the same, only the water’s temp. is different. The use of immersion suits for such a short fly (to my opinion) is an over kill. I think that if you are worry about the plunge make sure to have jackets and a dinghy. Make sure to have an ELB with you. My a/c is fitted with one and the wife and I are fitted with one each.