There is no answer to this really. It depends not only on the enclosure used but also the ambient humidity of the area you live in as well has you temps.

If you have trouble keeping humidity you can

Switch to a substrate that holds humidity better (Repti-bark, Cypress mulch etc)
Have an additional water dish on the warm side
Have a humid hide
Partially cover the top of your enclosure
Etc

Here is a good thread about tank setup.

http://www.ball-pythons.net/forums/s...ad.php?t=56846