Only you know your concentration level, but I know that for me, 2 hours was my limit with lessons (or films, or most things for that matter). I had a one hour lesson before my test (where suddenly all my maneuvers fell apart, but thankfully came together for the test). I think if I had had a 2 hour lesson, I would have been wiped out for the test, or worked myself up into an even more nervous mess.
As I mentioned before, doing the test routes worked out great for me. Granted, I took the test in an area where I don't live (back at the in-laws, rather than in London), so it really helped with recognizing speed limits, non-inuitive intersections, bike/bus lanes, etc. Also, there was one very tricky roundabout which was on most of the routes. I must have gone around that thing 20 times before my test. I also happened to get a test route that I had practiced last thing the day before. Bonus!