If you allow an hour a leson up to lesson 12 (first solo) and then add a few extra hours to get the hang of hovering etc you get up to the 18 - 20 hour figure everyone has mentioned. Any earlier than this is just pointless and potential for disaster.
A main point I have a slight grumble about is that should the engine fail when a student is early solo they have not yet (following the sylabus order) progressed to forced landings andvanced autos or any real learning of EOL. Leaves you rather stuck when the donkey stops having only had a short demo.
I personally continue on with students letting them solo in the hover after initial circuit solo to help with confidence (short periods not an hour at first liable to fatigue and mistakes follow) However I continue on to cover advanced autos etc. This also helps to boost their confidence when they start to realise they can cope and remove some of the fear of being up without the instructor. This then promotes faster learning and means they finish in the same time as an early solo student.