A good quality commercial lawn mower should easily last between 4,000 to 5,000 hours before it needs any major repair or replacement of the lawn mower.
You should be charging enough money on your lawn mowing jobs to pay for the mower quickly and then be able to put enough money aside through the years to pay for a new commercial riding mower should you need one.
You do not want to have trouble with your lawn mower when trying to run a good lawn mowing business because trying to save money with your broken down mower or mower that frequently breaks down will cost you money and even jobs.
Most commercial lawn mowers that see a lot of use rack up the hours within 3 to 4 years and after 5 to 6 years you should be looking at getting a new or nearly new commercial lawn mower.