Just because a seal is rated for 250 or 300 degrees this may only be for intermittent service. Again, check with your supplier. Chemicals in the environment around the unit may not be compatible with the seals, caustic wash downs for example. Airborne abrasives like sand, dirt, paper dust, weld slag, salt spray and even lint can stock to the rod and start to deteriorate the seals. Look at special rod wipers or possible rod boots.
Seals can deteriorate due to being incompatible with the media being used to power the cylinders.
Double-check your specifications. Another form of abrasion to the seals can come from the piston rod itself. The process of hard chroming steel for piston rods is the standard in the industry. When the cracks cross over another crack there becomes a pinpoint of steel that is exposed and will rust in the right atmosphere. These points of rust become small sharp bumps on the rod that cause excessive wear.