Never pull off your fan leaves. Who told you to do that? The plant needs them to produce energy but also to respirate.

Take those and it can be death in hours.

The flowers get all the light they need at that stage of their lives, or else they wouldnt even be there to begin with.

Trichs form as the flowers start. The longer they flower, the larger the trics get, and the more numerous they become.

During the last couple of weeks is when the fill with the most THC.

The older trichs degrade first. I am not sure the trich has a 'lifespan' as you put it as it really isnt alive.

Trichs will last several years after being harvest so they dont die.

The THC in the trichs will degrade in by products while still in a living plant.

I would probably estimate they will turn from clear, to cloudy, to amber in approx 6 weeks for most indica strains and maybe 8-9 weeks for sativa.

Just generalities, but if this is because of the fan leaves you want to remove I wouldnt do it.

Hope that helps.