Go by your maintenance schedule published by your manufacturer. Keep an eye on the color, when it starts to lose its red color property and turn brown it is time to flush the system.
〉 Answered on Apr 30th, 2009 by Amanda J. Pierce, MBA "AJ", Certified Mechanic, Project Coordinator; Primavera Scheduler at VIGOR Alaska Industrial Shipyard & BizzM3ch Solutions
It should be changed every 30,000 miles with regular driving. If you tow or haul heavy loads regularly, I would recommend you change it at 15,000 mile intervals. As always, refer to your owner's manual.
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〉 Answered on Apr 30th, 2009 by Karen Davis, Service Manager at Smith Stokes