Be very patient with version 12.1 update.

I’d suggest you may be having connection issues of some type. Are you running a wired, or wireless system?

It could be wifi interference or possibly a duplicate IP address issue that’s causing the device to disconnect and reconnect repeatedly, slowing the update process. 

I certainly agree, 20 minutes is way too long. But I’m glad you stuck with it.

As a precautionary measure, I’d recommend a refresh of your network, by unplugging all Sonos devices from power, then rebooting your router. Once the router comes back up, plug back in the Sonos devices. It might help, and certainly won’t hurt. 

Agree with @Airgetlam .  The update took the usual 2-3 minutes for me.  Glad it got there in the end for you!

To be honest, I’ve never really bought in to the idea of rebooting the router.  That’s just fundamentally wrong with Sonos speakers if that’s the case.  Imagine every time you add a device (whether it be a computer, smart home device, phone, tablet, and etc) to the network and you have to reboot the router to make it work.  That’s just not how network and network devices are supposed to work.  Duplicate IP should be a non-issue.  When was the last time you update your phone, computer or whatnot and have to ask the owner of the router to reboot because of duplicate IP address.  That’s just mind baffling.  The router and device should have protocol to deal with this kind of issue.  The fact that I have to reboot the router tells me the Sonos firmware is cutting corner somewhere.  That should be better handled.

While I agree with the idea that it shouldn’t be an issue, I’ve had too much personal experience with routers not properly releasing IP addresses so that you run out, or handing out duplicates when a device (like Sonos does) soft reboots. Frankly, a monthly reload / of the router’s software has always been one of those housekeeping things for me. Perhaps in buying standard home wifi gear, rather than professional grade is a mistake. But then again, I reboot my computer periodically.

The difference between a Sonos device and most devices on your LAN is two fold. Most devices on your LAN don’t soft reboot and request new IP addresses, they just stick with what they have, where as every time you update the Sonos software, they soft reboot and request new addresses. Second, each Sonos device does substantially more ‘between device’ connection than anything on your network, as they communicate to maintain sync. 

For what it’s worth, the responsibility of the handling of IP addresses in the current networking world is completely on the router’s side. All any device is able to do is request an IP address from the router, and use that IP address. If the router hands out a duplicate, the only thing any device knows is it’s getting interrupted when attempting to communicate with the router. Sonos has cut zero corners here, they’re just exposed more frequently because of the frequency of updates Sonos does. 

The most common way around this issue is assigning reserved IP addresses, again in the router’s DHCP table, so that the possibility of being handed a bad IP address is negated for those devices. This is fairly standard ‘good housekeeping’ for networks, but most home users don’t realize it.

Finally, another advantage or rebooting the router is that it forces it to pick up any potential DNS changes that may have occurred upstream, and then pass them to the speakers that are rebooted. Granted, DNS changes don’t occur frequently, but I can assure you from personal experience that when they do, they can cause havoc. 

But it’s up to you. You get the opportunity to choose how mindful you want to be about your own network, with the knowledge you have. You do not need to listen to people like me.