timing belt?

when should a timing belt be change on a Peugeot Partner Escapade Combi [long name] MPV 2004 2.lt
37,000 miles since new just over 6,000 per year not main car but used for towing caravan.
I have been told with past other makes 60,000 miles on most. Local Dealer has sent warning letter that change is over due saying it should be done every 5yrs but I thought it was gauged on mileage!.
Thanks in anticipation.

bob
December 2010