When is the right time to change car tires?

You're kinda on your own once you get past eight years, especially with wide temperature variations (like you might find in AK) between at-rest cold and at-speed warm. Otherwise I just use the penny trick for checking tread depth and be done with it.

dogpile.com before asking here

I've never used the penny wear test before, I just observed it with my eyes. Thanks @ Elvis Telecom and everyone. I found out this article about the penny used to check the tread depth, https://tireer.com/penny-tire-test/ it is really helpful for me right now.

Always check your tyres before you drive. There should be about four.