Depends how many miles you plan on doing in the next year. My personal preference is 5 years/ 60k, as this means you can change the plugs at the same time and save a bit of labour cost.
V6 belt failure is less common than with TS's, which need to be changed every 36k.
Note to all V6 owners: nearly all the V6 belt failures I have seen have been caused by wear in the tensioner, but this can often be spotted before it fails. You should inspect the belt at least every 12k to ensure that it is not starting to run "off centre". As the tensioner wears, it forces the cambelt to start running at a slight angle, causing it to run close to the outer edge (and eventually off the edge) of the other pulleys.