I have two credit cards (through the same bank). The first I got at age 18, and the 2nd I applied for while in grad school for better rewards. I use credit cards (primarily the more recent one) for routine monthly expenses and I pay them off in full every month and reap the rewards. The limits were pretty low while I was in school, but every 6 months or so they would notify me that they were raising my credit limit by a modest amount. This continued for a while, but it stopped not long after I left school (which roughly coincided with banks becoming more reluctant to offer credit).
My income has increased several hundred percent over what it was while I was in grad school, and I feel my credit limits do not reflect that. My combined credit limit between the two cards is a little over 12% of my yearly gross income. I feel this is probably low, and I might like to increase it both to increase my credit score and to easier allow for multiple large purchases (computer, furniture, etc.) in the same month without using an excessively high portion of my available credit. What guidelines should I use to determine how much of an increase, if any, to request?