Feeding dog How much?

I have a one year old X Sheppard, ( She is currently about 65 pounds) she has been ill for the past few months, and it was recommended that we feed her canned dog food. Recently she wants to eat non stop - she can eat two big tins of dog food a day is this too much? Not enough? Should we try supplementing with dry food? She is a farily active dog, on the skinnier looking side. Suggestions?