I really don't run mine in the cold or snow but when it is chilly outside I usually let it idle for as long as possible. If the block heater is needed and it is your main winter ride I would let it idle for at least 3-5 minutes.

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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