Spark Plugs


How much life have people gotten from their spark plugs?

I've got something like 4000 miles on my '02 and it seems a bit sluggish in the low RPMs. Perhaps a symptom of fouled plugs? Before I go tearing into them I thought I'd ask those who know more than me if 4000 miles is weirdly soon for spark plug replacement.
The manual states to check them every 4000 miles(6months) and to replace at at 7500 miles(12 months).
I bought a set yesterday , got 6300 miles on it and I feel they need to be changed. Plugs at the local Zuki shop were 4.90 each.
Sure wont hurt the bike none!
Well after posting the last message I decided to change the plugs. They looked pretty good and appeared to be the right heat range, bike has 6300 miles.
Well, I would highly recommend changing the plugs at this mileage, the bike seems to run a whole lot better than it did !

First time I had the bike apart also , wasn't as bad I had thought it would be ;)