I could make mine last five to seven weeks. I would have anywhere from two to three rows, going back. I kept my braids tied up when in the house, anytime I pulled a shirt over my head, layed down, or slept. I loved my cornrows.
But...I didn't shampoo once a week because I didn't want frizzy braids. So, I would go in between my rows with witch hazel or a cap full of shampoo in a bowl of water to clean my scalp then follow up by using a natural oil between the rows. I shampooed & conditioned every two to three weeks. After a while the itching does stop. But, my problem was anytime I sprayed something on my cornrows to keep my hair moist, my scalp begin to itch. So, I stopped and the itching stopped. I kept my cornrows in back to back from February 05 until a week ago. Needless to say, my hair was very dry. If it wasn't for the dry hair and the fact that my edges on my right side were starting to disappear, I would probably still have them in.
I would suggest that if you want to keep you hair moisturized while in braids, get box braids. Den1 shows you how to do this step by step.