Assuming you start the bike weekly and alow the motor to reach operating temperature for a 20 minute period, and you are not storing the bike next to a stream, once a year should be fine.
Reposted for my amusment:
I use a different type of oil every time I change, always 10w 40, except in summer when I run 15 w 40, unless I cannot find it, then I run 20w 50,unless I have some 10w 40 around, I just use it, even if it is not a "different" type of oil. So, I guess my original statement was more of a general concept, not a personal commitment.
![Roll Eyes ::)](http://forums.sohc4.net/Smileys/default/rolleyes.gif)
BTW:
I re-polish my oil filter housing every time I change the oil.
I add a little Sea-Foam to the oil about 100 miles before I change it, unless I forget, or the mood to wrench & polish hits me and I just change the oil.