I think I did an hour a week, although I would say between 60 and 90 minutes once or twice a week would be a good balance - it's a lot of cognitive load initially until you reach some level of "Conscious Competence" (i.e. you can concentrate on the traffic ahead instead of trying to to find the right gear!) https://en.wikipedia.org/wiki/Four_stages_of_competence
Intensive can be good, but only if you experience all the different weathers in that week! Over a longer period you will get rain, ice, night time, busy traffic, light traffic etc.
As for @pads comment - the Finland driving test covers advanced driving such as drifting, handling cars on ice, snow and various other techniques. There's a reason they have bred many of the greatest Rally Drivers. I'm sure they would fail on the M25!
I would recommend (if you don't do it as part of your course), doing a couple of hours on a motorway. After passing I asked my instructor for a couple of hours of motorway driving experience (in the end we turned back after just over an hour). It was good as he was an ex-copper and knew a lot about advanced techniques and reading ahead. Motorways are a step up from dual carriageways!