Assuming you are referring to the smell of ozone produced by an air purifier, it will typically dissipate within 30 minutes to an hour. However, this is dependent on the size of the room, the type and size of air purifier, and the amount of fresh air circulating in the room.
If you are instead referring to the smell of ozone produced by thunderstorms or other electrical activity, it should dissipate relatively quickly once the storm has passed.