I just replaced mine (late 58 plate car) as I am on 60k miles now. Garage said that it was a good job I did as the idler bearing was failing, which was overheating the wheel and melting it.
If you read the service manual it says to replace at 120,000 miles but then in the small print at the bottom it recommends replacement every 5 years, or 4 years for colder climates.
I would have said 5 years, 60k miles is as far as you would want to push it.
As stated, I had mine changed by a local indie specialist and for belt, tensioner, idler, waterpump and aux belt all in was a little over £280.
Average price though is about £350.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso