It’s going to vary quite a bit from car to car, but the number I’ve heard the most is about 2 minutes for a typical car. Not sure about bikes.
As for the OP, if you’ve got an old fashioned carburetor type car then the fuel at idle is determined only by the set screw on the side of the carburator. It doesn’t make a hill of beans worth of difference if you are braking or using the engine to slow you down. The amount of gas flowing into the carb will be the same in either case. On a fancy shmancy computer controlled car, it’s not so clear cut. I’m not familiar with how the car manufacturers program their computers, so a lot of this is guesswork, but the computer is going to monitor the idle speed and adjust the flow of gas accordingly. At some point you will use less gas if you use the engine for braking instead of the brake, but at some point I would think that the computer is still going to force some minimum amount of gas into the engine, even if the engine is idling at an acceptable speed without any gas (due to the intertia of the car pushing the engine around). Still, it seems to me that overall you’d have a small fuel savings using the engine to stop the car instead of the brakes.
Note - all of this is looking at the problem only from a fuel efficiency point of view, and ignoring all practical considerations about why you may or may not want to do any of the above.