Ideally, as long as possible. The longer you inhale, the more time the gas has to work in your body.
When inhaling at your recommended duty percentage, it will take roughly 20 minutes for your body to become saturated with hydrogen, at which point your body cannot contain more hydrogen. At this point, if you continue inhaling, you’ll keep your body saturated with as much hydrogen as it can hold. After you stop inhaling, it will take another 20 minutes for the hydrogen to dissipate from your body.
Some people can experience discomfort when they start inhaling. If this occurs, reduce the amount of time you inhale the gas to the point where you don’t feel that discomfort. Over time, you can gradually increase the length of your sessions. Once you can do a 20-minute session, you should be ok to increase your times as much as you want.