It depends more on the climate where you live than how often you play.
If you're in a very dry area, I'd say every 6 months or so, but if it looks and feels fine, don't bother until it starts to look a little dry.
In a very humid area less often, maybe once a year, but it's really a matter of just judging the wood.
When I was working as a tech for Paul Plumeri I NEVER oiled the boards, only when the frets were done on a guitar did it get any oil.
Paul didn't seem to believe in oiling the wood so I had to do what he wanted, his guitars after all. :-)
Just as an example, Martin guitars suggest NEVER adding any oils or any kind of treatment to their fretboards, just keep them clean. So it's all up to you how often, or even IF you oil your fretboards.
Most rosewood boards stay pretty healthy without oil, but I prefer to oil them about ever 6 months or so, just to be on the safe side.
YMMV though. :-)