How long do tire plugs last (and how reliable are they really?)

If you've got a puncture in your tire, you may look to plug the hole so you can get back on the road without a new tire, but how long will it last?
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Ask Honest John - Are self-sealing tyres any good?

I have been looking at car tyres and some of the big manufacturers, Bridgestone, Pirelli, Continental and Hankook offer self sealing tyres. How good ...