I have a 2015 Honda rancher that had a crack in the air box. It got so much dirt through the air filter it got into my piston. So it now

Burns oil and I was wonder will it ever stop smoking. I know I have to rebuild the engin so don't even say it. And how long will it take before it losses power.

You should stop running it now before it causes bigger problems that make it ruin the engine block because then you'll need a whole new engine and it's impossble to tell how much life it has left

The damage will not heal by itself. It needs a doctor for surgery, not medicine. The more you let dirt in, the sooner and worse the engine damage will be. Organize your tools so, you won't need to start and stop during this extreme job. Bore and hone the cylinder, install new pistons with rings, and do a valve job.

No it will not stop smoking - you know what you have to do so I won't say it. It may run a while fouling plugs and needing oil added all the time- if you fixed the air box it could smoke for quite a long time and may not wear any more.

Your motor is burning oil because you don't change your oil ad filter every 3,000 miles.
All that happens with out an air filter working is a dirty throttle body.
Have you been getting your valves adjusted every 60,000 miles? Honda's are known for getting tight valves and they need to be adjusted that often.