Normal blood sugar for a nondiabetic dog is pretty much like normal blood sugar for people. It falls generally between 75 and 120 mg per deciliter (mg/dL) [Merck Veterinary Manual].
Anything significantly below 80 mg/dL is in the low or hypoglycemic range and blood sugar lower than 60 is a crisis, especially if it is likely to drop further.
Blood sugar that persistently exceeds about 180 mg/dL will cause the body to spill glucose into the urine [Merck Veterinary Manual]. See your veternarian!