marrt,
If those hyd cylinders were done correctly, they should last a good while. However, if a nick or cut occurred when reinstalling the piston assembly, or wrong o-rings, or incorrect direction of some seals, would give a short life. Did you rebuild those cylinders yourself, or have them done. If a shop did them, they should have tested them at some higher pressure than what is normally used, and state that on the bill. What kind of fluid are you using in the hydraulic system?
Someone mentioned about the o-rings broken and some pushed out of the groove. What would happen in that case, is that the pressure of the fluid would try and push the piston over to the side, further compressing the o-ring, and allowing more fluid to pass by, and the particles smaller than 10 microns would be wearing down spots on the cylinder wall. If an inside tube micrometer were used to measure the tube for true roundness, you might have cause for obtaining another cylinder. I have no idea what the tolerances would be.