You can call AMEX after 6 months from the card issue date. They will give you 2 options based on ur profile
1. Limit increase without any docs - usually less than <50% of ur CL
2. Send your income docs to them - based on ur docs they will increase the CL
PS: Don't call on weekends (Sat,Sun) and public holiday, the CL increase team would not work on these days