Some of it can be absorbed directly through the tongue, I believe. I would think the longer you can hold it, the more is absorbed, but maybe there is a limit, so maybe more than 30 seconds isn't going to do you any good? Just guessing, really.
From Healthline :
"Your healthcare provider usually orders sublingual or buccal medications when there is a need for the medication to be absorbed rapidly. This type of medication may also be ordered if the effects will be lessened during the digestion process. The cheek and the area under the tongue have a lot of capillaries, or tiny blood vessels. This means that the medication can be absorbed directly into the bloodstream without needing to go through your digestive system."