Most of the times I use a vaporizer so I have to grind my weed up pretty finley. When I do I usually grind up a few days worth at a time. Nothing more.

If your going to grind get a spacecase weed grinder off ebay. They have a little screen at the bottom that lets you catch all of the crystals and after a while you can just smoke those. They won't bash the shit out of your weed like a coffee grinder will. Plus you don't have to worry that, 2-3 oz down the road, your Pop's going to grind up some beans, along with a shitload of extremely potent thc crystals, for a nice pot of coffee before the big meeting with the boss.

As far as losing potency is concerned, yes, it will happen. But you'll never know it. Seriously, the amount of crystals that will come off might lower the potency of a joint by something like .00000001%. So, unless you have some super human ability that nobody else does don't worry about it.