in the beginning, you dont even need 20-30 minnutes. It all depends how good shape you are in though, regardless, HIIT is likley to be humbeling.
Id start with 15 min 2 times a week. If its all good, do 3 times a week. Once you get in better cardiovascular (sp?) shape, upp the time. if you do TRUE HIIT not like 5 min run, 5 min jogg. but INTENSE intervals, thats likley all you need because you will be pretty shot after it. I tried 1 min fast 2 min slow for 15 minnutes on my first session, and it absolutley killed me! (though i was in pretty bad shape then)