Tires plugs will last the life of the tire when the tire plug is properly installed.
A Tire Plug that is installed correctly is a permanent repair although tire patches are much better.
Tires can be plugged more than once as long as the holes are not too close together.
You can plug a tire twice if you get a hole in the tire more than once.
However patching a tire is much better but when done properly a good tire plug can hold and last until the tire needs replaced.
I've plugged tires 4 times before and the tire lasted until it needed replaced at 50,000 miles.
Just never put a tire plug in a sidewall as that is not safe.
if the tire has a hole in the sidewall it should always be replaced with a new tire instead of trying to plug the sidewall.
I have however had to in an emergency plug the sidewall of my own truck tire because I had no spare with me and was on the side of the highway and needed to get off the highway.
But I did go get a new tire installed within that same day.