Hopefully never! But of course its a little more complicated than that...
Ozone molecules are constantly being produced and destroyed by different types of ultraviolet (UV) light from the sun. Usually, the production and destruction of ozone is balanced, so the amount of ozone is stable. You can think of the amount of ozone as the water level in a bucket with a small hole in the bottom and a hose adding water at the top. When you turn on the water, you will find a balance point where the amount of water in the bucket stays constant - the hose adds exactly the same amount of water that the hole removes. Thats how ozone is created and destroyed in most of the world, but unfortunately the Antarctic ozone hole is so severely depleted that it no longer exists in this perfect balance.
We first noticed that the ozone layer was thinning over Antarctica in the 1980s and scientists were able to identify several ozone-depleting substances used in products including refrigerators and aerosols that were adding chlorine and bromine to the ozone layer, which effectively widen the hole. This means that the process between creating and removing ozone is no longer in balance and there is more ozone removed than created. Since we cant make more ozone, the solution is to reduce the hole by producing less chlorine and bromine containing substances. To return to the bucket analogy, this will have the same result as narrowing the hole in the bucket back to its natural size.
The good news is if we stop producing these harmful substances the ozone will not only stop being further depleted but it actually will be able to recover and we will never need to worry about all of the ozone layer disappearing.