I usually do 3-5 in primary, 15-20 in secondary, and drink it as soon as it's carbonated in the keg (usually 24-48 hours). I don't like aging in the keg, I can't keep myself from dipping into it anyway.
These sound like good times for each. I like to cold condition my beer in the keg for a week to 10 days. This helps drop any chill haze you may have. However, as everyone has said, bitters are meant to be drunk "young" so as soon as carbonation occurs, you should be good to go.
The way I do it is ferment in glass until terminal gravity is reached.. transfer into a keg, carbonate and drink. The beer will change in character as it ages.. usually by the time you get to the bottom of the keg, it's a little more 'mature' tasting, which can be considered an improvement.. but it's still darn good when it's fresh. This is a good reason to brew 10 gallon (or 15 like I do) so that you can experience it at different ages so you can determine what you like best. You may also want to consider 'dry hopping' your bitter by putting an ounce of an aroma type hop in a large tea ball.. put that right in the keg and carbonate.. yum.. have fun..