If you go to Karwa driving school, the minimum lesson is 7 hours. Many of my friends at work went to Karwa and I have helped a friend register there as well.
Even though I took my license at Al-Rayah (minimum 12 hours) but I would recommend Karwa as it is a very organised school. The only downside is maybe it is slightly a bit out i.e. in the outskirts of Doha (near Abu Hamour) - but then that depends on where you are staying.