DVSA quoted averages are 45 hours with an instructor and 22 private practice.
Roundabouts are probably what most people fear the most due to their complexity, but with the right instructor you'll master them in no time. Build up the basic skills, junctions etc. and then moving onto roundabouts shouldn't be too daunting. You will not (shouldn't) be thrown in straight onto major, busy roundabouts on your first lesson!