Depends on the plug....
If its just the ol string with black goop stuck in the hole
then thats not a good solution....
There is a new plug that is very good and I have used it and
had no problems... Have a screw in the tire now and will go
and have it fixed this weekend too.
The plug looks like a upside down mushroom, with a wire coming
out the stem. The nail is pulled from the tire, a drill is
used to size and shape the hole and the wire inserted into
the new hole from the inside of the tire. Tire has been preped
cleaned and glued. The wire is grabed by pliars and the plug
pulled flush with the inside of the tire. The extra length of
the plug is cut flush with the tread and the inside of the tire
is hot rolled to complete the process.
So what you end up with is a plug through the hole and a hot patch on the inside of the tire.
The tire has to come off of the wheel for the installation.
Seeing this installed on one of my tires, I don't see how this
type of plug could ever fail.