Time between service is going to vary greatly depending on riding conditions. Dry, clean roads and low to moderate mileage might put you on a yearly schedule whereas wet or dirty conditions or high mileage could neccessitate an every other month service schedule.
I would recommend you buy one of the popular bicycle maintenance manuals. Overhauling and adjusting a bottom bracket is so much easier than I thought it would be.
I'm assuming you're talking about putting oil into the bb shell through the cable guide screw hole on the bottom of the shell? Don't do it. The oil could thin the grease on the bearings making them more susceptible to washing out and wearing down. Just take that screw out after a wet ride to drain any water in the bb shell, but don't put anything in.
>"I don't mind a little adjustment once in a while with the Dura Ace BB".
However, most mechanics would suggest maintenance much more often than "once in a while" for the DA BB. Hence, unless you are a pro who has a mechanic to do it for you most days, the vast majority of riders would be better off with the Ultegra BB.