I would do nothing less than two hours. Two and a half if possible. Experience has been a great teacher for me.

Fortunately, there are (I believe) at least two additional LAX flights on US after the one on which you are ticketed, if there were a connection problem.

On another less Philly-bashing note.......if you get out of FCO in time, there's every chance you could be a bit early getting into PHL (my last experience with US #3 was almost the same day last year and was an hour early). Then you'll sail right through.