That's a it depends matter. Chain is measured by stretch which is directly relayed to the weight handled and good maintenance and conditions. Leave it in the rain all the time, lift max weight loads and never lube the chains will give you a short life...
Mast bearings etc should last a long time UNLESS they are damaged or abused - dirt and debris in the mast, overloaded, forklift used to push instead of carry etc, (the bearing is designed to go up and down, not transmit impact from pushing). A good operator, and regular maintenance will make these last for years.

The other items that are changed often in a mast are teh pivot bearing (where the mast joins the body)anything to do with sideshift, forks etc. As noted by many folks take care of the lift, and your maintenance costs will be a lot lower