I know that you’ve probably heard a lot about it. The useful life of the asset is the period over which it’s depreciated into expenses and the period should always equal to the period the asset is actually going to be used. So in short the estimation should always equal to the actual usage. 

Do you have many assets with their carrying value at zero? Have you actually looked at it? Matter of fact is that if you’ve got too many assets with their carrying value at almost or at zero, it’s a strong indication that your useful lives are way off. That’s obviously depending if those assets are actually to be written off at sometime soon or are waiting for reinvestment or replacement and are depending on the availability of funds for investment.

Such monitoring should be done at least once in a year and actions to make determined useful lives closer to actual expected period in use should be undertaken accordingly. It may seem like it’s only done for accounting, but if you think about it – shouldn’t expenses be covered with income earned in respective periods? If you depreciate assets faster than you earn income from them, your revenues are not meeting expenses, namely they’re in wrong periods.