Question
How much shade can a cedar hedge withstand if properly watered. Pesently I want to plant a hedge under a row of large Maple trees and there is little sun. I live near Kingston Ontario which I believe is zone #4

Answer
As to shade, a cedar hedge will need some sunshine.  If it doesn't get at least 5-6 hours, it will get "leggy" (the fancy term is photropism).  Actually, it is also the QUALITY of light that it is missing out on.  Red light is better for leaf growth, and blue light is better for flower production.  

I suppose that if the plants get reflected RED light part-time, during the day, that would help the situation (i.e. planted next to a big, red barn).

In the wintertime, when the leaves are off of the maple trees, they would get some light, but not if they are buried under 3 feet of snow!  Here in our area, that is a factor, as snowy weather doesn't persist, and evergreens do a bit of growing in the winter months, if properly watered.

The Cedar tree (Calocedrus/Cedrus) is a forest native, so the juveniles have had to compete under the forest canopy for sunlight.  They compensate by growing quickly.  If you were to hedge the trees down, they would probably do OK for the first few years, but then fail to perform well later.

I think you & I might also be talking about a DIFFERENT Cedar (not the California, Incense, Deodar, Himalayan or Blue Atlas - All of which have to grow in a zone 5 or better).  Maybe there is a variety that does well in Ontario that I don't know.

Here is a list of Evergreen hedges that I know can tolerate HEAVY shade and grow in a zone 4:
Japanese yew (Taxus cuspidata)- Medium growth rate
Dense yew (Taxus x media 'Densiformis')- Medium growth rate
Canadian hemlock (Tsuga Canadensis) - will get 50 feet IF you don't prune it down!

You'll have a leaf-litter/maintenance problem from the Maples above, that you'll have to pluck out of your hedge every year!

So, here are some zone 4 Deciduous shrubs that can be hedged, and can handle heavy shade (and drop their leaves, right along with the Maples!):
Winged burning bush (Euonymus alatus) - Awesome fall color!
Slender deutzia  (Deutzia gracilis)

Hope it helped.  ~M