How long you cook the pork depends on how you would like the pork done. 30 minutes should give you a lovely rosy pink medium-rare, which is now accepted to be safe for pork (internal temperature of 145°F as measured with a food thermometer). If you have any concerns, or simply prefer your meat a little well done, you can cook a little longer, or until the internal temperature reads 71°C / 160° F on a digital thermometer.