What she said, pretty much. As to how long it takes to break down... Do you mean physically, in your mouth? It depends on how much saliva it comes in contact with. More saliva increases speed but also increases the risk of some or all of the product being dissolved in it and sweeped away into the rest of the mouth and/or swallowed. So personally I try to leave it relatively dry under my tongue. Then it takes up to 40 minutes, but in average maybe 25 minutes.
If you mean inside the body, as in, how long it takes to disappear from the blood, its half-life is 12 hours, I think, meaning that after 12 hours, the blood level should be half what it was at its peak, which is why taking HRT 2 or 3 times a day makes sense. Personally I take it twice a day, and I definitely feel the up-and-down effect, some days. So when I get a decent dose (which should coincidentally be divisible by 3), I'll split my daily doses in 3, so I can have something more stable.