There isn't any general agreement on how long a full spectrum light should be used - the recommendations range from 1 hour a day to 12 hours a day. 2-4 hours seems to be the most common recommendation. Personally I go for about 7 hours a day. The sticky thread at http://talkcockatiels.com/showthread.php?t=27549
might help you decide.
To get the UVB benefits, the light must be placed within 2 feet of the bird. Make sure there is a shaded place where the bird can get away from the glare if it wants to.
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