When my pickup was my daily driver I had the heater on a 1500 watt timer. I found even at 10° below, 2-3 hours of heat was enough for easy quick starts. What was your neighbor's reason for no more than 5 hours? Like posted, doesn't hurt a thing.
Heavy truck and diesel mechanic (thankfully retired after 30+ yrs)
'91 F-250 SC 4X4, 5spd. ATS Turbo, 3:55 diff.