Following are selected disclosures from the Rohm and Haas Company (a specialty chemical company) 2007 10-K

(in Millions)	2007	2006
Land	$146	$142
Buildings and Improvements	1,855	1,729
Machinery and equipment	6,155	5,721
Capitalized interest	352	340
Construction in progress	271	218
Land, buildings, and equipment, gross	8,779	8,150
Less: Accumulated depreciation	5,908	5,481
Total	$2,871	$2,669

The principal lives (in years) used in determining depreciation rates of various assets are: buildings and improvement (1050); machinery and equipment (520); automobiles, trucks and tank cars (310); furniture and xtures, laboratory equipment and other assets (510); capitalized software (57). The principal life used in determining the depreciation rate for leasehold improvements is the years remaining in the lease term or the useful life (in years) of the asset, whichever is shorter.

Impairment of Long-lived Assets Long-lived assets, other than investments, goodwill and indenite-lived intangible assets, are depreciated over their estimated useful lives, and are reviewed for impairment whenever changes in circumstances indicate the carrying value of the asset may not be recoverable. Such circumstances would include items such as a signicant decrease in the market price of a long-lived asset, a signicant adverse change in the manner the asset is being used or planned to be used or in its physical condition or a history of operating or cash ow losses associated with the use of the asset . . . When such events or changes occur, we assess the recoverability of the asset by comparing the carrying value of the asset to the expected future cash ows associated with the assets planned future use and eventual disposition of the asset, if applicable . . . We utilize marketplace assumptions to calculate the discounted cash ows used in determining the assets fair value . . . For the year ended December 31, 2007, we recognized approximately $24 million of xed asset impairment charges.

Question:

Rohm and Haas reported depreciation expense of $412 million in 2007. Estimate the useful life, on average, for its depreciable PPE assets.