When i plug tires, i take a pair of snips and snip away the excess plug that sticks out.
I also pour rubber contact cement all over the area and let it dry overnight before airing up the tire
All the plugs i have done myself have lasted the rest of the life of the tire.
I plugged one of my OEM goodyears at 9,000 miles. The tires now have 40K miles on them, worn down to nothing and the tire still holds air fine. I never have to air up at all. I can still see the plug too.
Guess i'm just good at plugging tires