We operate our legacy Citation (an early 80s model) just over 350 hours per year for the same cost as 200 hours (1/4 share) on a NetJets Ultra.
Those numbers are from October 2007 and do not include debt service on our current airplane OR the acquisition cost of the quarter share ($2.2M)...but as fuel has risen increasing the cost of operating our own airplane I'm sure NJA has increased its hourly fuel surcharge.
I'd say up to 100 hours per year chartering is the best way to go, between 100-200 hours a fractional share is the best option, and more than 200-225 hours per year operating your own aircraft is most cost effective. Of course, typical mission will play a major role in the aircraft you want and/or need...everybody wants to fly in a big airplane but many, many corporations and individuals have found themselves "airplane poor" after getting something far more capable than their needs dictate.
There are many, many pros/cons to both full and fractional ownership...be sure to carefully perform due diligence on both options before making a decision.