-get a a tread depth gauge (about 3-5$)
-rotate tires every 5-10,000 miles (10k is ok)
-find the 'born on' date codes on the tires...
when tread depth is 4-5/32nd start shopping tires...
OR if tread wear is uneven with areas down to the 3/32 nds indicators.
5-6 years from 'born on date' is time to shop tires...
or sooner if signs of degradation are seen.
tires that have been patched/plugged should be watched,
and balancing may be needed BEFORE replacement time.
consider adding a tpms,
they are a reliable means for monitoring leaks or sudden pressure loss
(many threads of tpms)
this thread documents 3 generations of tires as they roll from new2oblivion...