What most people regard as "stable" molecules can have lifetimes of anything from a few minutes to millions of years. Of course, the stability of a molecule is largely determined by the environment it finds itself in. Some molecules are instrinsically unstable, which means that they only exist for a very short time. In some cases, molecules with lifetimes as short as 10-15 seconds have been successfully characterised. This is such a short lifetime that such species are thought of not as molecules, but as "transition states" for the conversion of one molecule to another.