I recently updated to Android Studio 2.3, and now when I try to run the application, the emulator does not come online. It times out after 300 seconds.
Additionally,
Check you don't have the deviced unauthorized, unauthorized devices reply the same error in the Android Studio, check the emulator once is on with the adb command.
$ adb devices
List of devices attached
emulator-5554 unauthorized
If you have of this way the emulator the Android Studio is waiting for be authorized and maybe this can solve the problem.
Authorized Devices
This is a error I have solved in Windows 10 with Android Studio 2.3.3