I've only replaced my Spark plugs once, both at around 100,000. Both cars were used when I bought them so they may have been replaced before. I don't intend on replacing them again for another 50,000 or so miles. Checking their gap fairly regularly is a good idea, though.
As for your misfire, it could be simply a bad plug, your plug wires, a coil going bad. I'll freely admit that I don't know much about turbos but maybe that may causing some stress for your spark plugs too.