How long should my Green Iguana bask in the sun?

  How long does he need to be in the sun if his UVB bulb broke? I am buying one in a few days but he really likes to be in the sun. How long does he need the natural sun to be healthy? He has been out there for about 40 minutes and he is loving it.
They should be taken out of the sun when they begin to keep they're mouths open or need to be moistened with water to prevent dehydration but I have juveniles so it really depends mine only lasted in the sun about 20 minutes or so.