A protective layer of conformal coating will indeed expand the life expectancy of an electronic board that is then exposed to the working conditions, however a number of other parameters also play a role that needs to be considered.

For example an acrylic will have a reasonable performance however if outdoors the life expectancy maybe greatly by organic pollution that may affect the coating layer, while a urethane coating will resist most contaminants yet it will be very much affected by sunlight, oxidation and thermal cycling.

A silicone will resist most environmental attacks although direct abrasion will shorten the life expectancy. Only an integral approach in selecting the adequate conformal coating will provide the highest chance of electronic survival for each system (Otherwise only there would be one conformal coating in the market). This makes very important for you to work close with your CC provider to help you select the ideal conformal coatings for your application.

Most brands if not all, will offer tutorial and selection tools on their websites. The best solution again is to work closer to the experts to get the optimum selection.

It depends on the relative humidity of the environment,..drier is better. The board surface will absorb water relatively quickly (minutes to hours).

The extent of absorption is influenced by % RH and temperature. Cool and dry is the key, by all anecdotal sources I have talked with.

Boards can be kept indefinitely in room temperature and humidity conditions as conformal coatings are breathable and will allow moisture pass from the board through the coating.

A coated board can for example be kept at elevated temperature for several hours and moisture will still be removed from the FR4.

I'm sure this is part of the IPC specification. However I have had interns run tests on PCB FR4 and found a measurable weight difference over a 2 hour period and a 45mm square, 0.5mm thick board.