2 x 2 Hour Slots
There is a whole bunch of overhead burned up in start, taxi, pre-takeoff checks, waiting to takeoff, transit to practice area followed by the whole thing in reverse. Yes, you do have to learn all that lot properly, but it's all repeated every time you fly.
A longer slot gets you proportionately more time on the lesson material and can get you through the syllabus more efficiently with less total expenditure.
The most prolonged and costly method is flying a single hour once a week.