To keep your Fox air shock in the best condition you need to regularly clean and lubricate the air sleeve. If you are good with this and do the routine as regularly as the manual suggests, your original seals will last a long time and will simply need a wipe and inspection before re-installing. However, if you let things slip you're gonna need to replace 'em more often than that if you aren't going to wear expensive hard anodised parts!!

We normally reckon you should get between 6 and 18 months of use out of a Fox air seal kit before needing to shell out for a new one.

If you are the type who doesn't do much cleaning and maintenance:
seal kits cost £10 (2000+ FLOAT seal kit) and include all the air seals you will need for the complete air sleeve overhaul and a 5cc FLOAT Fluid capsule good for two lube-ups! Instructions for the air sleeve clean/lube routine can be found in your manual.

If you are the type who does the maintenance when told to:
Go straight to the top of the class, your seals will remain good until you get the damper serviced (every 12 to 18 months) where you will get Mojo to install the air seals, oil seals, nitrogen seals, recharge your shock with fresh oil and nitrogen and press in a fresh complete set of eyelet bushes and stick on a new shock sticker all for a song. Prices here.