Idealy baring injuries and contract problems or any thing else that might get in the way? I say 4 defenses every 12 months is ideal. 3 is fine as well. You need to take training and recovery into account as well.
I think most champs have to make a mandatory withing a certian period of time. A champ should defend his belt twice a year as long as he's healthy and politics do not prevent matches.
I would like to see champs take one fight per season, with 2 stay-busy fights and 2 meaningful fights