As I understand it there are various stages of breathing changes in someone's final hours. The last stage involves a strange sounding rattling in the breathing (caused by un-swallowed saliva at the back of the throat), hence the term 'death rattle', this is when death will most likely happen within the hour, (I am talking people here by the way as opposed to animals). Eyes will most likely have dilated pupils and the eyelid may remain slightly open even though the person is unconscious. In my late Husband's case he also started to form significant brown patches on the whites of his eyes.
A day or so prior to death the person often becomes alert and seems like they are much better, even having conversations with loved ones. It seems after this they go downhill very rapidly and are gone with 24-48 hours.