Depending on how often I ride, but I also check them anytime a wheel is off since it's quick and easy enough... But for a good reference, about twice a year is good. I pack it in with the same old grease in palm method.
The grease I prefer is the Amsoil Synthetic Multi Purpose Grease, I use this on just about everything because it's good and the price is good since I sell Amsoil as well. http://www.amsoil.com/shop/by-produc...?code=GLC3P-EA
They do carry higher grade greases and special racing greases which are much better as well... This stuff, the Dominator series synthetic grease is really good and it can take just about any abuse you can throw at it... http://www.amsoil.com/shop/by-produc...?code=GRGCR-EA
and I'm going to switch to this next time I need to order grease.
Any of the Black Molly greases will do an excellent job and same for the red stuff, but since I sell Amsoil, I also use it and it works great too.