In general, you want to buy a forklift that still has somewhere between a fourth and a half of its life left. That's probably somewhere between 7,000 and 11,000 hours. Of course, if you can find a machine with even fewer forklift hours, that is ideal, but you may not get the level of savings you're looking for.
The average lifespan of a forklift is 10,000 hours. It's also important to mention that it's common to see lift trucks last more than 20,000 hours from higher-quality brands, such as Toyota.