Shy,
If the unit is rebuilt properly and is up to spec, then it should not be failing every 10-20 hours, unless you have other problems.
If you have firewall, engine mount, or nosegear problems, or are using improper landing technique, then your shimmy damper may keep on failing even if you replace it new, every time.
In a nutshell, what you have is a shaft that's attached to part of your nosegear, and a shell of the damper that's attached to another part. Inside the damper, the shaft is also attached to a disc, or fixed "piston," with a calibrated hole in it. Oil in the damper must pass through that hole to allow the piston to move in the shell. The ability of the unit to resist passage is what contributes to the damping action.
The shaft and shell are sealed with several different packings and rings. Unless your damper is being operated in a very dusty environment, these packings and rings should last quite a while. Internally, if there is corrosion or wear, the unit may be bypassing or failing to properly dampen any shimmying tendencies. If the mounting bolts are loose, or wear exists where the bolts go through the dampener at the attach points, then shimmy action may also take place, because the unit can't be held solidly enough to do it's job.
A good place to start is a close inspection of the nosegear itself. Is it damaged in any way? Are the wheel bearings adequately lubricated and in good shape? Check them carefully, both the bearings and the races, and ensure that the packings, felt, and cups that go adjacent to them are installed correctly.
Does the axle align through the forks properly? Is the axle properly secured and torqued? an improperly secured nosewheel will shimmy.
Is the wheel true and in spec? A damaged wheel can shimmy and can also easily fail...is it properly torqued together? Is the tire in good repair, and the entire wheel assembly properly balanced? Balance is important, and it needs to be checked and done each time the wheel is removed...at least every 100 hours if not more frequently. I
s the nosetire air pressure proper? Low pressure can cause these problems as well as causing binding in the nosewheel forks, excessive shoulder wear, and excessive temps...and even an explosion. Check the mains, too. If they're off, and the aircraft is pulling, this can exacerbate other problems and assist in a shimmy.
Is there play when the wheel is off, between the nosegear forks and the upper nosegear? Is there any play in the scizzor linkage? This is a very common point for shimmying, as well as an aircraft that won't track straight on the ground. Is the nosegear assembly properly lubricated? Is the nosegear strut properly inflated, and serviced properly with H-5606 fluid to the proper level? If that strut is too high or too low, it can shimmy.
How about landing technique? Is that nosewheel contacting first at all? Pushing forward after landing? Pilot technique is probably the #1 cause of nosewheel problems, including most of the wear that the entire assembly, or any part of it, experiences.
Check the firewall for buckling. The inside of the nosewheel forks for wear where a tire may have been compressed on a hard nosewheel landing...it sometimes splays out and leaves black or grey marks inside the forks, as evidence. Check the engine mounts for damage, bending, cracking paint...expecially the lower supports. You may be able to see it by checking the space between the prop spinner and the cowling for evenness above and below the spinner, or right and left, too.
Check the steering rod ends for play; another common wear point, and a source for shimmy.
Your shimmy damper may be fine...you may have other problems. If it keeps leaking like a sieve, then you have a bad damper or a bad rebuild. If that isn't the issue, if it's still shimmying and you can't seem to stop it, you may have other problems than your damper, or you may have a worn out damper internally. Without seeing the damper, it's very hard to say what you have, and not enough information has been provided to lend much comment.
Are you running a nosegear fairing? What condition, if so? When does the shimmy occur? Why is the unit being rebuilt? Is leakage occuring? Can the shimmy be stopped by the pilot? How often is your nosegear lubricated, and with what? Are you running retreads? Do you balance the nosewheel assembly? A lot more information as needed, as you've by now guessed. Check on it, and post back.