From the relation between visibility and snowfall intensity chart in Canada
heavy snow fall is
In darkness
-1C(30F) and above : Visibility less than 1sm
Below -1C(30F) : Visibility less than 3/4sm
In daylight
-1C(30F) and above : Visibility less than 1/2sm
Below -1C(30F) : Visibility less than 3/8sm (1/4sm for FAA)
and for the heavy snowfall and some other precipitation, there is no holdover time guideline.
http://www.tc.gc.ca/tdc/projects/air/f/tables.htm
Oh and I've sprayed 2800L type I and 700L type IV on the 737-700. It had 3ins of ice and lots of snow on top of it. It took me more than 45mins to deice it. That's about CAD$19000 on the 737 and 3 of them all by myself with no canopy
.