Coolant change intervals? Is it based on time or mileage?

[SeafoodSC]

I know you're supposed to change coolant regularly to preserve headgaskets among other things, but are you supposed to do it at certain mileage intervals, or over a period of time? Reason I ask is because my SC only gets driven ~1,500 miles a year and sits in the garage all the time. I drive it only a few times a year, but take it out for a 30 min ride every month to keep it fresh. I haven't changed the coolant in 3-4 years, but it looks like its new still.