depends on your code base
in my experience,
very first android built can take time, sometimes 5-10mins(dependency download, gradle update etc)
very first ios build also take time but not as much as android
to give an another perspective, i have two macs, 2015 intel macbook pro(4th gen i7) and M1 macbook pro 16 inch, i have reasonably big flutter projects, but build time was never been an issue or concern) on both computers. but android studio and emulator sucks on intel mac.
after that, build doesn’t take that mich of time, if you are not clean-build, it always incrementally builds, so it is fast.
clean builds will take more time.
but if you are planning to do mobile apps, then get a mac. you need a mac to test and deploy ios app(even if you use flutter)