Using an automatic will speed things up as you don't need to worry about the use of gears and clutch, but according to the DVSA, the average time taken is around 45 lessons with 22 hours of additional practise if you can get this.
As an instructor, I have taken someone from the first lesson to a test pass in 13 hours, but this lad worked on a farm and was a natural driver. Other people struggle and take far longer. There is no easy answer to this I'm afraid as it depends entirely on the individual.