A dog leg amputation surgery takes around 30 minutes to an hour.
The dog will be put under anesthesia which can last for 3 hours before the dog wakes up after the surgery.
Dog amputation surgery is usually done faster than a humans amputation surgery because there's less they need to really do to get the dogs leg amputated.
Still it's a complicated surgery even for a dog to have it's leg amputated but the dog as long as it has it's 3 other legs can usually walk and run and get around just fine even without the 4th leg.
I've seen dogs with 3 legs run around like they never had an amputation of the leg.