I've only ever had one tyre plugged (to date) and decided I would change it at first sign of pressure running low or any hint of high speed 'wobble' or instability. Never had any probelms and ended up running the tyre for another 2k+ miles before replacing as tread was low. My tyre fitter mate had a good dig around at the plug when he took the old tyre off and declared it 'well and truly plugged' after all those miles and about 6 months.