In general, it's best if you have done everything you know to do to make it finished before letting anyone else see it. However, in my experience, it depends on what the beta is reading for.
I have a beta who is spot-on with character, world & plot development. I can give her a chunk of story (usually all the same story arc) and ask if I've really mucked something up as I fear I have and can't see. She can give feedback at any point in the writing and brainstorm with me as needed.
I have another beta who can't see problems in chunks and has to have the entire story to work from. She's ace at spotting inconsistencies and unanswered story questions I've left hanging. In truth, what she does can only be done on a finished draft.
I have one last beta whose specialty is grammar. She can spot line edit problems in a glance AND she knows my style well enough that she can tell if I purposely broke the rule, most of the time, and then tells me whether or not I succeeded. She only gets the very last draft before I'm ready to submit.
I've worked with each of these ladies for at least 10 years. We know each other's strengths in beta reading. We share the same concept of what a beta reader should do and we have established a good working relationship.