Here's how often you should take apart and service your Aeron chair:
Never. Ever.
I have one at home, partially to compensate for the crippler they have set me up with at work. It's used - off craigslist of course - and I was having problems with one of the arms not really wanting to move side-to-side.
The problem with a sample group of two is that you can't be quite sure which one is busted. Turns out, it was the other side. A flat piece of injection-molded plastic was mashed, and that meant it didn't do its little bit of magic quite right.
Understand that where I live, you can't turn left without hitting an Aeron. They're everywhere. Thank the dot-com boom or something. There's a billboard on the way in "Silicon Valley: Turning America's 401Ks into chairs." So it was quite a surprise to find no online information at all about servicing them. No parts, either. I just needed what would be a $5.00 part, and even that would be at grossly marked-up prices.
No dice. There's pretty much squat. And service-wise, this may be the only geek fetish item that doesn't have two dozen unboxings and disassemblies spread across a hundred odd fan sites. There's nothing. There's not even Aeron pr0n!
Undaunted, I proceed as usual in these circumstances: do nothing for a month or two.
Then, looking to leverage my success, Project Aeron was joined with Project Mold-Making For No Reason. Another month passes with similar results.
Until today:

The original part is on the left. RTV silicone meets Quick Cast. 24 hours later, I have a very nearly identical-looking part, with questionable physical properties. Strangely, it went without a hitch.
I wish I could say the same thing for the reassembly. In my class on product design, there are different focus areas, like DFM (Design for Manufacturing), DFE (Design for Environment), etc. These chairs were created with DFSYOIYTIA (Design for Screwing You Over If You Take It Apart).
I have met many products from that school of design. They usually involve a spring under tension or compression with a couple of degrees of freedom that are enabled as soon as you undo any part of it. This one has two.
There are things that should self-destruct when you try to work on them. Cryptographic machines. Atomic weapons. Waterbeds. But not chairs. Particularly, not expensive chairs that you might need to repair.
Yet there they were, the two magic springs, freed from the prison that had held them captive and useful for so many years. Those little bastards.
The internet has nothing on how to reassemble these.
Anyway, I could blab on about this for another couple hundred words, but long story short: You can get them back together. Attention internet: You can get them back together! The technique requires vice grips, a screwdriver, and a sailor's-worth of salty language.
x2 if you're doing both sides.
UPDATE: After four hours or so in service, the replacement cast piece has shattered deep within the workings of the chair. This is unfortunate, but not surprising. What's even more unfortunate is that I will now be pursuing a broad survey of all of the casting materials available in an attempt to find the right one. After all, I have that awesome mold now!