The instructions that come with most masks say three to five times a week, ten minutes a session. That's a reasonable place to start and most people never need to think about it again.
But the question behind the question is usually either "can I speed this up?" or "am I doing it wrong?" So here's a fuller answer.
Can you speed it up by doing more?
Not meaningfully. Doubling the session length doesn't double anything, and using it twice a day is mostly a way of getting bored with it faster.
Light therapy is one of those things where the dose curve flattens. Past a certain point you're spending evenings for no additional benefit.
The cadence that actually works
Three or four sessions a week, ten minutes each, attached to something you already do. That's roughly forty minutes a week, which is nothing, and it's sustainable for months, which is the entire point.
If you're the sort of person who does things properly for a fortnight and then stops, aim lower. Twice a week for six months beats daily for three weeks by a distance.
When to skip it
If your skin is irritated, sunburnt, or you've just had a treatment of some kind, leave it. Nothing bad happens if you skip a week.
If you're on any medication that increases light sensitivity, that's a conversation for your doctor or pharmacist before you start, not a question for a brand's FAQ page.
Morning or night?
Whichever you'll do. The practical argument for evening is that clean skin without makeup is a requirement, and most people are only reliably in that state at night.
The practical argument for morning is that some people find it wakes them up. Neither is wrong.
What to do during the ten minutes
This sounds trivial and isn't. The people who stick with it are usually the ones who paired it with something they enjoy. A podcast, an album, sitting on the back step.
The people who quit are usually the ones who spent ten minutes staring at a wall feeling faintly ridiculous.
You will look ridiculous. That's part of it.
General information, not personal advice.