A few variables
G'day Rush,
At my airline (Turboprop regional in Oz) we must Deice if there is ice/frost/snow on the aircraft (in the opinion of the ground engineer OR the Pilot - i.e. to NOT deice, both must agree that it isn't necessary). The actual temp isn't relevant for us; it is the presence of ice, frost or snow that matters.
Having said that, we don't do it much either, as the weather in Oz is pretty good. There are probably only 2 days per year that it's required at any of our capital cities (frost). It happens a lot more at some of our regional airports, and we have two airports that service our ski fields in winter (Cooma and Mt Hotham) where it happens more often.
Other airlines may have different procedures.
I'll add this bit as well (Not quite what you asked for, but anyway).
Ice protection (anti-ice) is different.
We must put the Ice Protection on when there is either:
A): Visible moisture (e.g. in cloud or rain) and the outside temperature is 5 degrees C or less when airborne
B): Same as above but 10 degrees C or less when on the ground
C): If on the ground (again 10 degrees or less) and there is standing water/slush/snow which could get ingested into the engine intakes.
Cheers,
DIVOSH!