when is it suitable to use a saved Irradiance Map?

[czeca]

i've been looking in the manual and forums and I found no info about this...

Im rendering with Primary Engine: Irradiance Map, S.E.: D. MonteCarlo

My question is when is it suitable to use a saved map? in other way, what things can be changed and still use the same I.M.? objects, lights, lights intensity, materials,...

knowing when can I use a previous Irradiance map will same me lots of time when re-rendering

thanks