It depends on what temp you have trouble starting your engine. It is OK to leave on overnight. If you want to be picky about it just buy a timer at your local Walmart and have it turn on 1-2 hours before start time.

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R