This is not always easy and sometimes it may require expert opinion. One option is to have some standard useful lives for common assets. For instance you can say all motor vehicles should have 05 years of useful life.
As per IAS 16.56 All the following factors are to be considered in determining the useful life of an asset:
(a) expected usage of the asset. Usage is assessed by reference to the asset’s expected capacity or physical output.
(b) expected physical wear and tear, which depends on operational factors such as the number of shifts for which the asset is to be used and the repair and maintenance programme, and the care and maintenance of the asset while idle.
(c) technical or commercial obsolescence arising from changes or improvements in production, or from a change in the market demand for the product or service output of the asset.
(d) legal or similar limits on the use of the asset, such as the expiry dates of related leases.
