The useful life would be the period that the property can be depreciated over. In the case of a lawn mower, must be used at least 50% for business, is depreciated over 5 years. So if you use the 179 deduction and the use of the mower falls under 50% business use during the 5 year depreciation period you will have a prorated portion of your deduction to pay back.
The 179 deduction is a great thing, but you must use it over the depreciation period. If you are weary of getting hurt or losing business and owing money, you might look at using the 30% special depreciation allowance instead. You would deduct only 30% of the cost, instead of the full 100%, and then depreciate the rest over 5 years. Less money to pay back if worse came to worse.