I actually had my card since 2006 with an CL of $100. I never really worried about CLI until I noticed this year that I still had only a $100 CL. I called back in March for a CLI and they gave me $100, so now its at $200. How often can I request CLI? I've read that some people have called every couple of weeks and received CLI and then I've read that you may have to wait 3-6 months for one.
Best increase is when you are going to purchase something big or more than your current CL. They increased my CL at the cashier by $900 or 100%. This was about 2-3 years back.
I tried calling a few months ago for CLI. Told them I wanted to buy furniture online, but I was told to come to the store and get a CLI at the checkout stand. I had $0 balance when I called so probably the CSR thought I had enough available credit.