Ack!
Ack!
do it as many times as you want as long as you get the color consistency that you want.
once a day, once a week, once a month, once a year, or never.
Some people never do it and the color that comes out is enough to meet their expectations.
If you're not calibrating at least once a week if not more often, you're doing yourself a huge disservice, an example of this is once I had to match a job from a customer who's Canon CLC-1000 might get calibrated when the hardware tech was repairing the equipment (nevermind that the company's salesman told the customer they could get something ridiculous like 300 150p sales books with critical color duplexed on 60# cover over a weekend).
Due to color matching problems, and design problems with the equipment (toner blades, faulty duplexing set up even on text weight, etc), it took almost 2 weeks of constant running with numerous repair calls (techs ended up on 24/7 call) to get the job out, and even then they complained because the color wasn't spot on to their example from their uncalibrated CLC. They insisted that since they had a CLC-1000 and we had a CLC-1000 (different RIPs as well as I recall), that it should automagically match.
We have a DocuColor 260 where I now work and they calibrate weekly under normal conditions and per job when it's color critical. I've not heard many complaints yet, but at the same time, with the Prinect Workflow we have and the PM52, we're better able to put jobs where they need to be quickly, a distinct advantage over the previous place, which was still camera and shake and bake plates or service bureau to get film back.
Anyway, my recommendation is set up a schedule to calibrate, whether it be daily or weekly, make it the first thing on the agenda, and plan some flexibility in to where it can be repeated as necessary and still hit dealines.