If you mean by de-register taking it off the road completely (i.e. scrap the car), I would imagine it will take as long as it takes you to stand in the queue at your local traffic department and the time it takes to fill out the form. If the car belongs to the bank, that will be something different which you will need to sort out with the bank. If it belongs to the company (no residuals, lease, etc) then a trip to the traffic dept should sort it out.
If you sold the vehicle to a 3rd party, then it will take as long as the 3rd party takes to register it on his/her name.
