instead of long period of time, you can use phrases like:
a long term
a long spell
a phase
you can even use the word tenure if it suits the situation.
Another word could be era.
Regime is the obvious word. It would be nice to think of others.
Anachronistic.
the word war did not exist in that time period the prehistoric era was a time when the was no written documents
The word 'mummy' is generally used for a body which has been preserved over a long period of time. A recently-preserved body is usually referred to as 'embalmed'.
Analyze, monitor, keep track of
Another word could be era.
Dwelt- to get a hold of something for a long time. or you have hanged around that thing for a long period of time Don't now the other word for dwelt(sorry!)
AGE.
exposure time = induction period
For a while can be used to mean 'for a short period of time'.
exposure time = induction period
Age, era, period of time etc.
interval, lapse of time, period, span, stretch, timespan.
eon
Era Eon Epoch Forty days & forty nights Forever
veteren