The excitation lifetime of an isolated molecule is one thing:
and can depend on the number and type of states that it's allowed to relax to,
but it can also become de-excited by other non-radiative processes.
As to your other question, I have no direct experience with quantized light fields, so this may be an oversimplified idea, but in terms of what the photon is "doing", you can think of any excited state wave function as describing a atom/molecule + photon system, and what it's "doing" is making up the difference between the ground and excited states. I guess that doesn't really explain "what it's doing" but just "how to think about it". Hope that was at all helpful.