I usually buy a new mower every other year. I run 2 mowers, therefore, buying a new mower every year, alternating years. Anyways, I depreciate the whole expense over 2 years, the length that I have the mower.
It's the same with a truck, 4 years. However long I keep a piece of equipment (large piece, trailer, truck, mower etc.,) that's how long it gets depreciated for. Basically, you want to take the depreication for however long it takes to pay for the piece of equipment.
If you can pay for the piece in one year, take the whole expense. If it takes 3 years, take the depreciation for 3 years.