Michael - I'm not exactly sure but my best guess is that it varies.

Are you familiar with Marshall Swift - I put a link to their web site below
"Determine replacement costs and depreciation values of any commercial structure or improvement in the field with Marshall & Swift building cost data and methodology. Enjoy instant access to the most current material and labor costs, free technical support and special multipliers localized to your market."

Michael,
These links should give you the answers you need.
http://www.fixedassetinfo.com/articles/Depreciation_Power_Lu…

http://www.landlord.com/depreciation.htm

http://forums.biggerpockets.com/post-80006.html

http://www.irs.gov/publications/p946/index.html

You should contact a real estate tax specialist. Call Brian Cain at 858.277.2122 for assistance.