FFF applies browser_cache_age_limit, but you can set it higher or for
'all time':
## It's common for browser cached files to be kept for several hours
## or more. You can limit the age of cached files FFF will use from
## browser_cache_path with browser_cache_age_limit. Only cached files
## that were downloaded within 'browser_cache_age_limit' hours will be
## used. If set to -1, all cached files will be used.
browser_cache_age_limit:4.0
The age limit is a performance measure--you can easily have many
thousands of entries in your cache that FFF has to scan for story
chapter URLs.
Your browser has a cache size limit in it somewhere, but I doubt it will
be an issue if that's all your doing for that period of time. Text
pages are small vs images, movies, etc, and will be more recent.