How long does it take for a language to change so much that the current speakers can’t understand the old speakers?

I’ve answered a closely related question:

Nick Nicholas’ answer to How long would it take an isolated group of people to develop what would be considered their own language?

….

[Lots of provisos, which you must read]

A ballpark figure is going to be closer to between 500 and 1000 years. With all the provisos already given.