I've found most people get back to me within three weeks or so when they beta my novels (80k-ish words). As for me, I discovered Word has a feature that tells you how long you've worked in a document on your computer, so I know I spend around 12 hours on most beta reads.

Most people reply to me, and when they don't I assume they weren't ready for feedback. I've only commented to them once, when I saw someone I had beta'd for asking for betas for another project. I commented that he had never got back to me, and he apologised and said he'd forgotten.