If you drill a hole from front to back of a ball gag and run a narrow tube - 1/2 to 1/4 inch - through it, with an inch or two projecting in front, you can leave her gagged for hours or even days, as she can suck water or even liquid food through it without being released.

If you get really clever and do the same job on a penis gag, you can have the added pleasure of making her suck a cock for everything she eats and drinks.

But whatever shape the gag, make sure it's not too wide. Something she can stretch her jaws round for an hour or so may be far too big to hold long term, and you may find she can't swallow with her mouth too far open.