Whenever he is in range, lol. It is tricky, because he can WC through them. That's why I use them carefully. make sure they are either blocked or hit him. I meter burn them liberally, too. I let him know i am not afraid to burn his ass. Grundy hate fire anyway. Watch how much you gain from that pressure. Keep your spacing, walking back and forth. You wanna stay within jumping distanceat least, to punish Swamp hand attempts. Sneak in a standing 3 to Mid/Low pump shot when you wiggle your way in range of the 3. regular Pump Shot is surprisingly decent as Anti air, too.
If he tries to WC (and he will), jump back and Space Hook the grab animation (then reload nuke), Pump Shot, or Hook Charge (if you're living dangerously). You can nuetral jump and punish, but that depends on the Grundy player's level. He CAN cancel, then AA grab you, I believe.
What you want to do is put some kind of pressure on him at all times. Let him know you are a ranged threat. Too many players run from Grundy and they are used to that. Give them something else to ponder, I say.