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:
When light hits beer, it provides the energy necessary to drive a reaction that transforms the iso-alpha-acids into 3-methyl-2-butene-1-thiol. The “thiol” part of that somewhat cumbersome name indicates that there is sulfur present. Sulfur compounds often have strong, offensive aromas. Some musteline animals, like skunks, have evolved the ability to produce this chemical, and use it for self-defense.
The reaction proceeds quickly with clear or blue glass:
[...] a well-hopped beer in clear glass can become noticeably offensive with just 30 seconds of exposure to sunshine. Brown glass transmits less visible light than the previously mentioned colors, and therefore offers some protection from skunking. It does allow some light through, so beer in brown bottles will skunk after a few hours of light exposure.
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