Yep, this greatly depends on your machine and which target you're building -- there are builds and configurations that can do a full clean build in ~30 minutes, but that's a simpler config and a high-end machine.
For the workflow:
You should only need to do the full clean build once, after that incremental builds should be much much shorter (our goals are generally <<1min, but that significantly depends on what you change, and your machine). You should always try to use incremental builds.
And if you know that you only want to rebuild a single partition / don't need images / etc, you can ask for `m vendorimage`/`m sync`/etc, but what works there greatly depends on the types of changes you're making, and what you need out of the build.