How long does the macOS Sonoma update take on a 2019 MacBook Air?

There's no way to predict whether or not you would have any problems upgrading from Catalina to Sonoma. It would require intimate knowledge of your MBAir and even after ruling out anything obvious it would still be a guess.

As for the time it takes to upgrade, you should expect anything from about 45 minutes to 1.5 hours. Also, be aware that the screen will go blank (completely black) a number of times during the process and sometimes it may feel like it's never going to come back on. Just be patient.

A few more tips ...

• Only do the upgrade when your MBAir is plugged into wall power - don't do it on battery power.
• Disconnect all peripherals (drives, printers, scanners, displays, USB hubs, etc.) before doing an upgrade. An external wired keyboard, mouse or trackpad is ok. Don't use a bluetooth kbd, mouse or trackpad when upgrading.
• You may need need to update various apps.
• You may need to reinstall reinstall printer/scanner drivers.

you're welcome.

to set up an external SSD for use with an alternate macOS, you can follow the steps in this informative user tip: Use an external SSD as your startup disk with your iMac or Mac mini - User Tip. and please note that it says this works on iMacs and Mac minis, but it will also work just as well for your MacBook Air. however, I would recommend installing the macOS that you don't need to use as often onto the external drive.

as far as booting into the external SSD, all you need to do is, with the external drive connected to the Mac, you restart your Mac. then when the Mac begins to start back up, as soon as you see the apple logo on your screen, press and hold your keyboard's "option" key until you see the screen where you select which drive you want to boot into.

hard to give you an exact time as there are many variables, but a rough figure would be 40 minutes to an hour.

that said, you might want to upgrade from Catalina to Big Sur before trying to get to Sonoma due to the big changes introduced in Big Sur. and if you have the time, I would recommend upgrading one macOS at a time all the way up to Sonoma.

yes, it is possible to partition the drive and use Catalina on one partition and Sonoma on the other. but IMHO, dual booting from the local drive can be problematic. I too have an occasional need for Catalina. so I keep my iMac's local drive updated with the recent macOS, and have installed Catalina onto an external Samsung T7 SSD. then when I need Catalina, I just boot into the external SSD.

I'm running FMP 16 on Sequoia and also find it has lost some of its GUI capability. However, I've found that when I go to a preview layout there's no button to go back to the browser initial layout but when I hit the Return key it takes me back. The same goes when I try to use a print layout. No apparent buttons at first but the Return key sends me to the next stage where there are print or cancel buttons.

I't still looking for alternatives for FMP that are not as expensive. Wishful thinking I guess.

As far as dual booting I'd prefer getting an external SSD and installing Catalina on it. Then you can boot from it to run FMP. When I had my 2017 Intel 27" iMac I used a couple of these 1 TB setups to run older systems:

Very economical and the speed was more than sufficient.

If your Mac is critical for your work do not set System Update to be automatic. Let others test it out and you can check to see if any have problems with your critical software. Then update manually.

Sonoma, about one hour, including software downloading and installing.

When I updated my 2019 MacBook Air from Catalina to Sonoma, the whole process took about 1 to 2 hours, depending on internet speed and how busy Apple’s servers were. I didn’t hit any major problems, just make sure to back up your stuff first to be safe.

There is no certain time for an update to install. Depends on many variables. Just make sure to backup via Time Machine(or whatever you use for backups). Id suggest starting the install right before bed and just letting it run

you can find the other macOS downloads here>> How to download and install macOS - Apple Support. you can go to the "use the App Store" section of that article to download and install Big Sur.

If you have any of the following installed,"cleaning", "optimizing", "speed-up", anti-virus or VPN apps, uninstall them according to the developers' instructions before upgrading your software.