I just tried successfully (but only after conversation with CSR) on my Barclays Arrival+ that I opened in September. I had asked for a fairly sizable increase and got it.
Don't know if there's an overall best time to ask--I've read around here that six months is when many cards give an auto CLI.
Chase Sapphire Preferred $32k; CapitalOne Venture $30k; Chase Slate $30k; Barclaycard Arrival Plus $25k; Wells Fargo Visa $25k; Amex EveryDay $25k; Barclaycard Ring $15.5k; Lowes $10k; Barclaycard Apple Financing $9.5k; Best Buy $7k; Chase Amazon Rewards Visa $6.5k; misc. store cards
EQ 822; TU 820; EX 819
