Last year I did Red Sea south (St John islands etc) at the beginning of November from Marsa Alam. Near Marsa Alam sea temperature was around 27ish degrees while further south it was around 28-29.
I dived with a suit that claims to be 2.2mm full body (w/o hood). In reality my suit measures (with calipers) about 1mm thick and I doubt it is neoprene - it looks/feels more like a rash guard material and it is (slightly) negatively buoyant. Underwater I was ok. Top side though it was quite windy and few times I felt the cold - thanks God my space on the boat was the inner most hence protected from the wind.
In comparison after that trip I was in Aqaba for few days end of November and did 2 dives. Water temperature was already down to 25oC. I used a 5mm full body (rented this time) and I was again ok.
Sorry I don't have experience further into the winter for water temperatures there.
For me my preference is:
- 25+ degrees my full body rashguard (my 2.2mm suit),
- 22-25 my 2 pieces (long john) 5mm full body suit
- 20-22 I add a 3mm vest and cut dive time and number short
- 16-20, I add a pair of (3mm) gloves and cut dive time and number short
I haven't dived below 16 degrees. That's for 2 dives a day. For LoB I would add 2 degrees to these figures and I don't think I would ever go for a LoB in waters colder than 25 degrees or so.
You have already seen that cold tolerance varies a lot among people. I think that only by trial and error you can find your sweet spot. Keep in mind though: it is always better to be a bit over protected other than become cold underwater. For example you can always open your zipper to let some fresh/cool water enter your body to cool you down at least with wet suits or remove your hood etc. On the other hand being cold (close or past the point of shivering) is very disturbing at best and can get VERY dangerous very easily underwater. Any further tiny disturbance easily becomes a huge issue once you are cold.
I also don't know why diving 3-4 times a day effects choosing the thickness.
To understand this think of your body as as heat production and storage facility. You create heat by burning calories, then (on cold days) you try to preserve this heat by covering yourself appropriately (eg top side with clothes, underwater with suit). There should be a balance between the amount of energy you produce to the amount of energy you loose. The problem is that underwater (thanks to the water which is a very good heat transfer medium) we loose heat much faster than top side (air is so so for transferring heat). Multiply this by 3 or 4 times a day, 1 hour each time (typical dive count on LoB) and you understand that during LoB our body loses A LOT of heat compared to a normal non diving day. Yes food is usually plenty on LoB, but heat lose can easily get much higher than what your body can produce digesting those falafels. Thus protecting yourself from cold becomes vital. Being cold underwater usually means end of dive and given that it is winter and you will be on a boat (meaning conditions top side won't be much better) usually means no more diving for the day. Better be prepared.
I hope these help.
All the best