That depends on the formula used to depreciate the asset. Normally
depreciation schedules will depreciate different types of assets at
different rates, e.g. Computer hardware @ 3 years, buildings @ 30, desks and
such @5. Once you have the class of asset and their depreciation schedule
(no. of years) set up in a table, then you set up another table with the
time the asset was purchased, the number of months to this month, (using the
DateDiff function) and then its simple math.
e.g. Computer Hardware over 36 months; 18 months of use under your belt,
have the assets value should have been depreciated. Many gov't tax
authorities as well as auditors provide guidelines for depreciation of
various asset types.