The CoC for a FF sensor is commonly said to be 0.03mm. Half the sensor diagonal means double the magnification whichh in turn means a CoC of 0.015. Basically you decide though, what's your print size and viewing distance.
photosmart; aim your camera aginst something with a lot of fine details. Shoot away from largest to smallest aperture opening. Look at the images in your raw converter set to 100% view. Your image quality will start to degrade somewhere between f/4 and f/5.6. But is it severe and will it show in your prints?
By making your own experiments you'll find the treshold for your own needs. (target, print size, personal taste). I just don't use smaller openings but f/8 with the G1 and if possible I stay with f/4 or f/5.6.
There are many not concerned at all with diffraction and some claim they don't see it. I'm not that lucky. OTOH I don't see the great color differences between lenses some people suffer from.
/Jonas