Even if you're mainline-to-mainline, 30 minutes is really pushing it in some cases. If you're arriving at C24 in the back of a 757 and leaving from E24, even if you chock right on time, that gives you about eight minutes to run the length of Terminal C (because your onward flight will close at -10 before ETD). I've done it but it was extremely hectic... 45 minutes should be the minimum domestic conx time.