According to many Air Purifier manufacturers, you should run your air purifier 24/7. Well, if this is not practical for you, then you can always adjust it and use it as you wish.
The air purifier can keep your room clean up to 95%, according to many studies, but it can keep your room clean only when they are running. Also, if you have a home with an HVAC system, you might have slightly dirtier or more polluted air in your room. So you might want to keep your air purifier on all the time so you can have good quality air.
The HVAC systems also have filters to clean the air, but they are not as sufficient as the air purifiers. The air purifiers can provide you with the cleanest air, and you will feel the difference immediately after the first usage.