I've heard get them pretty early. This is because what an individual as an author things is a good idea, and its better to get told that at an early stage when things can be easily changed than late in the process when you have made the idea very intricate and it is much more difficult to fix things. This video is mostly about writing or programming video games, but it also applies to writing and in a sense when to find beta readers.
I had that problem. I have an issue where if I tell someone else about my ideas I lose interest and never end up writing them. So I tried keeping my ideas to myself and ended up writing a much more than I usually did, but my plot became so intricate and interconnected that when I discovered severe problems in my plot that could potentially wreck the story I couldn't figure out how to fix it without ruining the plot. All because I didn't seek out feedback early in the process.
That said, I don't know the exact relationship to have with beta readers. Beta readers generally don't appreciate reading incomplete chapters or at least things that can't be made coherent in context, but at exactly what point a chapter is considered worth sending is unclear. They do say be very considerate with your beta readers because they're almost invariably reading unpolished stuff that's harder to read through than a final product.