How often do you change engine coolant?
For modern vehicles:
Generally Every 5 years regardless of distance travelled.
That applies for pretty-much every vehicle I've seen in the last 10 years.
Read the manufacturers recommendations and then assess your particular circumstances.
Generally, if I've got a car that i'm unsure of it's maintenance upkeep, I'll go through and replace all such fluids, replace filters and give the car a good mech-check.