If you're using the Vanity Planet Aira Ionic Facial Steamer, a good rule of thumb is about once or twice a week. This seems to be a sweet spot for a lot of different skin types, letting your skin enjoy the benefits without getting upset or overly sensitive.
Using it too often could dry out your skin or cause irritation, so it might be wise to start slow and see how your skin takes to it.
Regarding the essential oils, the manufacturer says the Aira comes with three little baskets specially designed for them. This is neat because it helps you avoid mixing scents, which might ruin your relaxing vibe. You just need to soak the foam insert of the basket with your favorite oil and put it on the steamer's nozzle.
Just a heads up though, based on what we've found, oils can sometimes mess with the nozzle or make unwanted droplets if you're not careful. So, maybe use the oil baskets sparingly and give the steamer a good clean to keep everything running smoothly.
Some other pointers: always use distilled water (this helps avoid any mineral buildup), and don't overfill the water tank to keep things running smoothly.
Finally, a bit of maintenance goes a long way. Cleaning your steamer after each use, especially if you've used essential oils, will help extend its life and keep it in good shape.