Find out what your maximum heart rate is (google, lots of info and calculators) then during your high intensity shoot for 80-90 percent of that. During your rest period let it come down to 60-70 percent for a bit then continue to repeat. You'll be moving between your anaerobic zone to your fat burning zone. Times are less important in the beginning. You can switch back and forth more often then as you get better your times can increase.