You need to lube the chain every time you degrease it.
You need to clean and degrease the chain as often as required.
Which depends on what kind of lube you use? Some lubes pick up more dirt than others.
It depends on the conditions you ride in? Riding in the monsoons or off-road will require more regular cleaning and lubrication.
The best way is to keep an eye on the drivetrain and clean it as and when it looks dirty.
If your chain is slightly dirty and dry and you are unable to clean it. Then at the very least lube it before heading out for a ride.
Hope this helps 🙂