I dont believe there any time limit on seat belts. My mechnic hasnt said anything and my records indicate that mine have never been replaced.
I believe what the Feds look at in a ramp check are the dated manufacturer's labels, along with any obvious fraying or broken parts. The labels are supposed to be legible. I just gave away a set of wag Aero seat/shoulder belts that were somewhat grungy but serviceable, with a few of the labels legible (dated 1999) and a few not legible.
Last Christmas I bought a new bright red set to go with the rest of my interior.
http://store.wagaero.com/product_info.php?products_id=2426About $350.