Like forkingabout says it all really depends on the manufacturer and also the model of the truck on how often the hydraulic oil should be changed.
Years ago the standard was around 1200 to 1500hrs or 1yr on most models but since the manufacture of oil development has improved greatly over the years and now they are integrating synthetic into oils more the time intervals have practically doubled from what i've seen.

I checked in Mitsubitshi, Caterpillar, Hyster and Yale and their current intervals ranged from 2000 to 2400 hrs or 1 year (whichever comes first) depending on the model truck.

As for pallet jacks most of the ones i found did not have a change interval, it was basically a monitor and top off suggestion. But if you want to take the time to change the oil i would follow the others, change it at least once a year.

Your machines should have an operators manual on them and those manuals do have maintenance schedules in them telling you this information. If they do not have operator manuals then i would suggest getting them and keeping them on each machine.
They are generally kept behind the seat or in a storage box or compartment of some sort. If those do not exist then fix them and make sure your trucks are in order. ;o)