10 years is the shelf life I bet, manufacturers do not give a use rating since that depends on what you do with them.
The life depends on your use factors. Machines that use the battery to start are much harder on them then those that do not (Mac II, IIx, IIfx for starters with 2 batteries). If the machine is a soft start and you leave it plugged in I would think the battery will last 10 years. A machine left in storage will not last anywhere near that long.
If I didn't get 4 years out of a new fresh battery with the limited use I put the machines through I would be mad.