Typically in order to view a lot of endospores in a bacterial culture capable of forming such structures, one would allow the culture to grow for 4-5 days. why would one have such prolonged interval?

The bacterial culture is always allowed to grow for four to five days to observe endospores because the basic survival strategy exhibited by bacteria isn't mature enough to be observed just after inoculation of the media.

Also, the endospores can be observed very easily when the culture is grown in a uniform and confined intervalm of time.