depends on how offen your fouling plugs, and riding style, if you putting around alot, and alot of lower rpm riding it will foul faster, my black blaster got over 25 hours per plug, but i spent ALOT of time jetting and fine tuning the carb, and it was as perfect as a stock yamaha carb can get, BUT like i said ALOT of jetting and tuning,
and a clean burn oil helps, if your running lower quality oil you will foul plugs more offen, i run pro honda hp2, its a clean burn oil, designed for honda cr motors, to help prevent port port( power valves) from getting gummed up,
now.. my old 84 honda atc 250r i ran honda gn2 2 stroke oil, which is a non synth/ none clean burn, and properly jetted and tuned i would foul a plug ever 6-8 hours of riding,