15k miles with leaded gasoline, 30k with unleaded was the standard in the '70s. We have better spark plugs now, and cleaner-burning gasoline, so you should be able to go a long time on a set of plugs. If putting in new plugs stopped the misfire, it reinforces that upgrading your ignition system (as a whole) will likely knock down the car's seeming need for brand-new plugs to run without misfire.