Charitably guessing that if they don't know how long they'll need to keep airspace closed then you give yourself plenty of time and rescind early if necessary, as opposed to continually issuing extensions which could cause confusion.
They don't have a mechanism for doing that. A military base near me has had continuous flight restrictions for decades. Each notice lasts a few months (e.g. https://tfr.faa.gov/tfr3/?page=detail_5_8746) and before it expires they issue a new one.
