This is another of those "it depends" answers.

If you are really into history and walking through all the old temples and gardens, Nara could easily use a full day, particularly when you figure finding someplace for lunch and perhaps an hour or two in the museum. If you move faster and pick out only the major sites, you could finish up in less than a full day. The nice thing in that case is you can just hop back on the train to Kyoto and spend whatever is left of the day catching a few more sites there. The JR Nara Line trains run back and forth Kyoto-Nara-Kyoto (and other places) multiple times per hour.