For the CBT here, it is an 8am start - 2 hours in the classroom covreing the road laws, bike basics, riding basics, and safety. Then it is as long as you need on site until the instructor and student are both comfortable on the bike and confident that the road will be OK. Then a minimum of two hours on the road. There is no pass or fail per se. You either get the certificate or you don't. If you don't, it is not recorded as a fail. You keep going as and when until you do (or give up trying).
Here is a not very well organised syllabus from the guvmint: https://www.gov.uk/search/all?keywords=element+&manual[]=%2Fguidance%2Fcompulsory-basic-training-cbt-syllabus-and-guidance-notes