When I first started Aerodrome Control training there was a minimum experience requirement (MER) of 30 hours. I think you will find that has probably increased since my days and the current times are probably contained within each ATC units Unit Training Plan (UTP).
So it really will depend on where you will be doing your on-the-job training and which Aerodrome rating you are going for and what previous controlling experience you have.
I'm sure someone will provide you with typical times/hours to validation, or you could always give the individual ATC units a ring.
On the beach