What is the largest hole a tire can bear before replacement? Saw this question today. The tire in question has a 2cm long hole on the inside, from a knife. 

I sometimes get puncture flats from e.g. glass. Never as big as 2cm. Usually if they're less than 1/2cm, I just keep using the tire. Is this at all dangerous? At what size does a hole in the tire become dangerous?
There's probably not a hard and fast rule as to how big is too big. It's going to vary based on tire width, pressure, the thickness of the tube, etc. 

As someone mentioned in the comments on that other question, damage to the threads is the biggest concern. If the puncture goes between the threads like a sewing needle would through clothing, there's definitely no problem. If one or two of the threads gets damaged, probably still not a problem. At what point it's too many threads to be any good anymore though is pretty much a judgment call.

One thing that you can be sure of: If you put the tube in and fill it up, and you can see the tube from the outside of the tire; the hole is too big and probably needs a boot to get you home. And if it's that big, you need to replace the tire.