When is a good time to replace a serpentine belt

 

When the outside starts to get glazed and the inside ribs start to show signs of cracking.
Depending on environment and vehicle usage that could be anywhere from 50,000kms to 100,000kms.

 

Sas very obviously knows a lot about what he's talking about, so no criticism, but just a small caveat to this statement: new serp belts are made out of EPDM, Ethylene Propylene Diene Monomer, vs older Neoprene rubber. This compound doesn't show wear the way they used to in the idea of visible cracking. I've seen belts with 150k on them that still *appear* new. It's the ribbing that gets wider, they even make a little gauge tool to "inspect" the condition of the width of the ribs.

Personally I plan on changing mine at about 5 years (I don't drive my car very much, owned for 2 years and have 4k on it) but for those that do drive regularly I would recommend around 100k mark.

Just my 2 cents
- Shane