statutory useful life
Legal and accounting term for the number of years over which an asset is depreciated for tax purposes, as specified by law.
The statutory useful life of this machine is 10 years.
耐用年数 is the general concept of useful life, while 法定耐用年数 specifically refers to the legally prescribed period for depreciation.
Compound of 法定 (statutory) + 耐用年数 (useful life).