Determination of timescales using characterization methods
The absorption of a photon en route to an electronically excited Franck-Condon state typically typically proceeds in the femtosecond range.

The lifetime of an electronically excited singlet state mostly is in the range of several nanoseconds. In some cases, however, fluorescence lifetimes in the range of several tenth of nanoseconds have been observed.

In cases where the initial excited singlet state undergoes intersystem crossing to a triplet state, much longer phosphorescence lifetimes (seconds range) are observed.

In order to detect a transient, its lifetime must be longer than the laser pulse needed to generate it. 

Once you've populated a state, you may either use 


time-resolved absorption spectroscopy to record it's spectrum and the decay of the absorption at a particular wavelength over time or
use time-resolved emission spectroscopy to measure the decrease of fluorescence (or phosphorescence emission) intensity at a particular wavelength over time.