I was wondering this as well. I got approved for it last July as my first non-secured CC. I was happy with the limit of $1,200 I got. Come April, I applied for an increase and got $2,200.
3 days ago I figured I'd give it another try. I clicked on the link for increase and it said something like "Congratulations we can give you another $1000 ($3,200). Your credit was not pulled, would you like to accept?" So I took it. I clicked the link again because I was curious and then it made me fill out all the info such as annual income, blah blah blah. So I didn't go through with that. Definitely wasn't 6 months apart when I got two increases though.
Click credit line increase from time to time to see if they will offer you an increase without even pulling your credit!