i just got my new 370z three weeks ago and already i had to put plugs in both  rear tires who can tell me how long should i keep these pluged tires

As long as its just a simple hole like a nail hole, a plug is perfectly fine and safe.  I have run plugged tires hard in wheel to wheel racing on the track with no ill effects before.

A plug should not be used to fix cuts and big holes that tear a tires cords.

thanks i live in oklahoma so i will change them out for some all season tires when the roads start to ice up