The 2003 toyota echo has 58,000 miles on it. Runs okay.
I wonder if I should change the plugs, or wait a while longer?

Replace them with the same plugs. This should help:

https://www.askmehelpdesk.com/cars-t...tml#post617111

They're due at 60,000 miles. Read this post on spark plugs and you'll be replacing them before the weekend.

Changing spark plugs

Gas is going up again, <arrg> Squeeze every bit of gas millage you can with that Echo echo echo.
Plug it!