Skunky off-flavours are caused by ultraviolet light causing chemical reaction that transforms alpha-acids. Once this flavour is created, does it go away with time, and if so how much time? Or will it stay forever once it is present?
No, it doesn't go away - the photochemical reaction produces compounds that do not degrade quickly and are not broken down by the yeast.
Professor Beer writes:
The reaction proceeds quickly with clear or blue glass:
The thiol compound doesn't break down spontaneously, so in unadulterated beer, the reaction is irreversible, and detection levels in humans is less than 1 billionth of a gram in a 12oz beer bottle, so it doesn't take much exposure to light to noticeably skunk the beer.
The skunk compound is normally insoluble in water, but can be made soluble by adding hydrogen peroxide, baking soda and soap, which is often used as a remedy to wash away shunk spray - and tested on Mythbusters! But of course this can't be done for the skunkiness in beer.
The best course of action is to simply keep the beer covered and out of sunlight