I had just shy of 22k miles on my front tire and about 16k on my rear. both were plugged and have been for the past 112k miles.
The only reason I changed them was because my rear tire was slashed and my front wouldn't pass inspection. By the way after the inspection I put the old front tire back on and did a 5k mile trip without a problem.
IE plug it (use worms) and forget about it.