I jumped on the upgrade bandwagon quickly this morning and the entire upgrade took about 15 minutes on my M1 MBP 13". Presumably, you're experiencing the same

I jumped on the upgrade bandwagon quickly this morning and the entire upgrade took about 15 minutes on my M1 MBP 13". Presumably, you're experiencing the same download issues my work MBP is experiencing. It's been downloading for over an hour and is just barely past 50%. My Internet bandwidth is 10G real-world and un-busy, so presumably Apple's CDN is getting hit good.