CB changed it so that if you have ever "bought" tokens, your account will remain light blue forever - the number of tokens on your account (it can sit at zero) will not remove your "light blue" status. I have multiple light blue accounts that have zero tokens on them and have been with 0 tokens for excess of 6 months.
Funnily enough, I have seen this same topic brought up again and again in other threads
If you are a broadcaster and use your last token, and have never bought tokens, your account will revert back to grey. I have seen this occur, there is no "time" limit to this, once you spend your last token your account will turn grey pretty much straight away.