How long does it take for an oyster to produce a pearl that's worth anything of value?

It varies but it does take a long time--up to 20 years.

http://www.youaskandy.com/questions-answers/29-ask-andy-1960/14735-how-long-does-it-take-an-oyster-to-make-a-pearl.html

"Freshwater pearls can take between 1 and 6 years to form; whereas saltwater may take between 5 and 20 years. The longer a pearl stays in the shell, the more nacre that forms and the larger the pearl. Freshwater pearls can grow to be just as big as saltwater pearls, but they take less time to develop because the layers of nacre are not as compact as those surrounding the saltwater pearl."

As to the value of a pearl, it depends on the color, shape, size, luster, and what is fashionable at the time. Seed pearls, even though they are very small, are in high demand due to their use in bridal fashion.