Sage Stallone -- Dead for at Least Three Days

Sage Stallone was dead in his room for a minimum of 3-4 days and possibly as long as a week ... this according to law enforcement sources connected with the case.

Our sources tell TMZ ... Sage "lived like Howard Hughes" ... he often spent days in his room which was "littered with junk," including an enormous number of cigarette butts, beer and soda cans, and food.

We're told there were two drawers filled with various pill bottles, some of which were "huge."

Our sources say Sage was living like a hermit ... no one had spoken with him since a week ago Friday. Although the housekeeper had been there earlier this week, she had specific ongoing instructions never to enter Sage's room. In fact, we're told the housekeeper was told to never even knock at the door.

One law enforcement source called the room "disgusting," saying it reeked of various smells.

We're told Sage's mother called the housekeeper yesterday after being unable to reach him, asking her to stop by and check on him. That's when she discovered the body.

Law enforcement sources tell us it's pretty clear from the evidence they saw ... Sage's death was an accident.