All depends on the quality of the materials used, how often its used, where its used, and how you care for it.
I've heard of suits lasting for 14 years before being retired, and they were used quite frequently outside.
I don't know if that's the average, however. I guess it'd last just as long as any other article of clothing as long as it was properly cared for.
Here's a care sheet I scrounged up.
http://savageturtlestudios.com/caresheet.html