When you take your vehicle in for an oil change the oil change should be completed within 20 to 30 minutes but sometimes it might take an hour or 2 hours before the oil change is actually done.
When I've taken my car to Walmart for an oil change it sometimes took 2 hours before they actually got to the oil change job on my car.
It usually takes them around 30 to 40 minutes to actually perform the oil change but sometimes they'll be other cars and vehicles ahead of your vehicle before they can change your oil.
When I've taken my car to a less busy auto shop they had my oil changed within 40 minutes.
So with Walmart expect at least 2 hours or so and at a regular small auto shop or just an auto shop in general it should take 1 hour or less to change your oil depending on how many cars or other vehicles are ahead of your vehicle.