My Visual Studio 2015 install was still going after about 3+ hours on the Android SDK. It seemed like it had hung.
To figure out what is going on, I looked in the %temp% directory and there is a file called AndroidSDK_SI.log. Opening this log I could see that the install was still happening. The problem was how long it was taking. It sped up significantly after I stopped Star Citizen updating!
The Visual Studio installer could really do with a better feedback mechanism to let you know that something is still actually happening. Some animated dots aren't very convincing after several hours. Even an advanced GUI option to display the output of the currently executing log file in a text box would be useful. The Android SDK log was quite useful as it showed percentage of the download, download speed and estimated times.
Anyway, check it out if your Android SDK install looks like it is hanging.
Another option is the kill the SecondaryInstaller.exe in Task Manager if you really think it is hung. This is what is running your Android SDK install. You can try repair Visual Studio later to make sure everything is okay.