I did dual n-back for ~60 days. My scores improved gradually to levels that would have seemed rather difficult (beyond what I thought I could do) when I first started. Then I think I got really bored with doing it everyday and my scores leveled off. (It is a really boring exercise...)

Over the course of the 60 days, I developed some techniques that gave me more consistency -- this lead me to believe my brain wasn't getting better generally, as much as I was getting better at dual n-back.