Go by time, my man. If you manage to wrack-up a lot of miles in say, 3-4-6 months, that means you are doing enough highway miles to clean the oil and engine. If not many miles are driven, then you need a change to get cruddy oil from short trips out. Time intervals allows you to adjust oil for climate changes, like 10w-40 for summer and 5w30 for winter. You can do it at your convenience, like before or ON holidays or state inspection. Time.