It will completely differ on the basis on the size of the walk-in tub, your faucet and your body and how much it water it displaces. It is quite an easy process to figure out the drain time and fill time of your tub.
Just, use your current tub faucet, fill a 5-gallon bucket with water and check out how long it’s taking. Then you will need to multiply it by the number of gallons that your bathtub holds and then divide by 5 (for the 5-gallon bucket). And then you will need to subtract the number of gallons of water that your body displaces.