My Exmark manual said at 250 hours and every year after. With only 50 hours of use per year it worked out to going 5 season without servicing it. Spending an extra $60 for filter and oil at the start of each season thereafter is steep; but I do it anyway. I am also an idiot and buy only genuine Briggs and Exmark parts for my mower and usually do a full tune up each season. But hey, after 12 seasons of use it still runs great. This year I went a little further and decided to replace all of the fuel lines, fuel pump and fuel shutoff.
Sidetracked.... Anyway, the two main issues with maintaining hydraulic systems is the introduction of contamination and air pockets in the pumps creating cavitation. I'm always careful to do exactly as the manual says. You don't ever want to drain the system unless it is necessary to do a repair. And you must always thoroughly clean the areas around your filter, the filler cap and anywhere else your system is being opened prior to beginning your service.