About how much longer can I use Catalina?

I believe you will get several more years of use from that Mac barring some sort of hardware failure.

Many users are still running Mojave and even High Sierra on their older Macs, though they are experiencing some of the issues brought up by @ku4hx.

I tend to nurse a Mac to ten or eleven years of use before it gets to retire. Make a plan now to start saving so you can replace it before too long.

There's no time limit on use. The problems you'll face are potential lack of compatibility with apps as they're updated, loss of OS security updates from Apple and the inability of old browsers to open websites requiring newer browsers with newer features and securities.

I'm specifically wondering if there will be future issues working with apps like Zoom, Apple Music, and Google Drive.

As we are neither software developers not Apple software engineers, we cannot even hazard a wild guess. You need to ask the companies involved and most are historically very reticent about discussing future development plans with consumers.

As long as compatible browsers with security are available, you should be able to continue using it.

+1 👍🏽