A coolant flush isn't expensive if you do it yourself.
According to the service recommnedations by Toyota, our coolant should be changed every 30K. Assuming an average of 15K per year, that comes to about 2 years.
I think it also depends on what kind of conditions you're in, and how quickly the coolant gets dirty. I know I won't be letting my coolant go more than 30K, because of the intense summers we have here.