It really isn't a chemical reaction, it is osmosis which is a physical process. In regards to stopping it, the problem doesn't reveal itself till the damage has occured and blisters have formed. Hence if a 12 year old boat hasn't developed blisteres yet, it probably isn't ever going to, but without protection it is a crapshoot. The damage can be fixed at great trouble or expense by mechanically grinding out each blister and repairing them with fiberglass, resin and fillers just like you would with any other physical damage/delamination in fiberglass.
As far as how long it takes for water to permeate the gelcoat that is a hard question to answer. Think of it this way: Imagine a sponge sitting on a wooden table and you are pouring small amounts of water into it, how long till the table is damaged? Well it depends on a bunch of things like how thick is the sponge, how much salt is in the water and are you ever stopping and allowing the sponge to dry out again. Just because the surface of the table gets wet, the table doesn't get permanently damaged immediately it will depend type of wood and the particular grain structure of that board. (Hint: the sponge is your gelcoat and the wood is your fiberglass laminate)
