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.