Your plate goes on retention instantly, and you get a new tax disc, etc etc, your V5 takes around 1-2 weeks to come back with the old reg on.
Then you'll get your private plate retention cetificate about a few days later,
Then when your ready to put it back on take the new car's v5 and the retention form and it'll go on instantly, then just a week or so for the new v5!
I think it's about £105 though for the retention of the plate.