according to the autodata disc it should be checked every 36,000 or 4 years(whichever comes first) and replaced at 72,000 or 5 years.
as others have said though, because of the low milages involved it should be replaced due to age fatigue.
like tyres, if they dont get used they rot at a much faster rate than those that get used every day. dont know why but thats the way it is.
if you just bought it at 6 years old the chances are it wasn't done by the previous owner. it doesnt look to complicated so should be resonably cheap just dont go to a fiat dealer. while they know the engine, their hourly rate is equivilant to a mortgage.
john