Radiation time vs. half life period
In the context of photochemistry, radiation time or half life period are not used, as far as I know. 

The only relevant technical term i'm aware of -in that context- is half-life (time), which describes the time needed for the concentration of an excited species to decrease to 50% of the initial value, cf. S. E. Braslavsky, Glossary of terms used in photochemistry,
Pure Appl. Chem., 2007, 79, 293-465
[DOI].

For longer-lived excited triplet states in the gas phase, radiationless deactivation upon collision with a ground-state molecules will play a role. 

Thus, the lifetime might be correlated with the mean free path of the molecules.