Ok, so I went and consulted and old official guide, and these figures are ballpark, so get out probably a minimum of 20 minutes before and be prepared to wait possibly 30-45 minutes for it. But here it possible schedule based on the October 1942 Official guide.
Dpt:
NYP: 8:00
NWK: 8:10 (locomotive change, could add 10 minutes, maybe...)
Easton, PA: 9:49
Bethlahem: 10:09
Allentown: 10:20
Emmaus: 10:31
Alburtis: 10:42
Topton: 10:55
Lyons: 11:02
Blandon: 11:18
Reading: 11:32
Wernersville: 11:48
Wormelsdorf: 12:00PM
Lebanon: 12:32
Annville: 12:41
Hershey: 12:54
Hummelstown: 1:02
Harrisburg: 1:22
From there on out, I would consult an Amtrak Keystone service timetable.
Cheers,
Nick