We had lessons last week. Four hours is about maximum, and that included 30 mins rest in a slope-side cafe!
We started at 9, which meant the first hour was very quiet! I am sure if you speak to your instructor, he/she will arrange to meet you at same time and place each morning if you dont fancy the full four hours ski lessons. The only problem would be that if you gave up after 2 hours, and were a long way from a slope that led easily to bottom of mountain it could be difficult!