When properly cared for, quality work boots can last a long time, but it also depends how many hours and the conditions you work in. On average, you'll probably need to replace your work boots yearly. Leather work boots exposed to the outdoor elements without routine conditioning can quickly dry out and crack. If you're constantly out in salty, winter snow and aren't cleaning the salt off your boots afterwards, you'll see wear and tear much faster as materials dehydrate.
High quality work boots can handle hundreds of miles under normal conditions, and you'll end up spending less by choosing quality boots that last versus replacing cheap boots every three to six months.
When your work boots wear out, don't replace them with your old sneakers. Work boots are designed to protect your feet and provide support to lower the chance of accidents on the job site. Plus, when you fork out the cash for a pair of quality work boots, know that you're getting more than just footwear for your job. There's a ton of other activities work boots are good for.