How often do you need to replace the power steering fluid?

Once per year or more often if desired. The process following is inexpensive enough to do more often. In fleet trucks that our shop maintains, we vacuum the power steering pump reservoir every oil change; and, refill it. This method constitutes a partial fluid change. The cost to replace fluid using this method is about $15.00. Since adopting this maintenance strategy, we've replaced no power steering pumps in the last 3 years on any of our 100+ fleet trucks.