I've read in my research that 80% of the caffeine will be released within the first 30 seconds of steeping. Longer steeping will increase the flavor (too long can over infuse it to a bitter tasting tea), but not necessarily the caffeine content which is particular to the type and cut of tea leaf you're using. To increase the caffeine content in a cup of tea, use more tea leaves. To reduce caffeine content, choose green or white teas, use less tea, and cooler water.
Lee @ Lee's Teas