After every day of riding. Its not difficult, you put the factory supplied syringe to the grease point and push in. Thats it, its as easy as oiling your chain.
I would rock the large, with the adjustable headtube and the fact that mine wieghs in at 38 lbs which makes it flickable. I like the large cuz I race and its more stable and tracks better. You can call Yeti, the guys there are awesome but Im sure they will say the same thing. I like bigger frames for sure.