Oh, it is about a 3 year refresh rate.
First mulch-toss was in 2006. Latest was in 2009. I had 50cuyd done both times. Mud-free, dust-free, grass-free, and nearly maintenance-free backyard costs me ~$600/year. Not tracking in 95% of the dust from the backyard when two kids & two dogs go in & out? Worth. Every. Cent.
Oh, I don't have to water it, either.
Over the 3 years, I would move the mulch around from where it accumulated (fence lines, corners, etc.) and spread it back where it was thin. After 3 years, it was getting too thin. The PG chips are clean hardwood and will rot. The 50cuyd refresh over top of the original 50cuyd is a world-beater. I am now at ~6" depth and it just plain rocks.
If I could, I would opt for PG-chip-sized cypress or cedar mulch, as neither rot much and the cedar is insect repellent.
If you are in an area dryer than DFW, your refresh rate ought be lesser, meaning you ought to go longer before refreshes. Flip-side for wetter areas.
Compare to having sod installed & water.