How far back can you cut holly bush?

You can cut them back to almost nothing if you want. But wait until the late winter that way you will get the spring flush of new growth soon after pruning.

Not far enough

Serious question, can I literally cut them to stump, and if so, how long until they spring branches and leaves to look like normal bushes again?

In February one year I cut mine down by 2 feet or more and had only brown branches remaining. They greened up very well before the end of the summer. I wish I’d have just ripped them out.

Took a chainsaw to the roots of mine several years back when they were overgrown. They were all above 6 feet tall and only green at the outer edges. Everything inside was brown and no leaves. The next summer they were about 2 feet tall. Little balls of green. The next year they got past 3 feet. Some closer to 4 feet. Now about 6 years later they’re all huge again. You don’t have to go to the stump but I really had no other options. They were all twisted and no growth inside. Was planning on cutting back but they were a mess. They were that way when we moved into the house so it needed a good pruning.

American holly? You can cut them to the ground and they’ll sprout back up. I’ve cut tree size holly (more than 12” circumference and 20’ tall) and they just send out multiple shoots. English holly is a little more temperamental—I’ve had some sprout from total cut-down and others die at the root after leveling them.

Those aren't boxwoods?

Ah okay. I'm used to seeing holly with much bigger leaves.