Who determines the useful life of a business asset?
The IRS does. Each depreciable asset gets placed into an asset class, and each asset class has a useful life (also called a recovery period) associated with it.
For example, office furniture belongs to the Office Furniture, Fixtures, and Equipment asset class, which assigns a useful life of 7 or 10 years, depending on the depreciation method used. An car would belong to the Automobiles, Taxis business class with a useful life of 5 years, and so on.
Asset classes, recovery periods, and instructions can be found in Appendix B, which starts on page 98, of IRS Publication 946, How to Depreciate Property. You'll notice this publication is labeled 2015, but the IRS has provided updates for 2016 and this is the guide to use.
TurboTax will figure out which asset class and recovery period applies to your asset.