If you have downloaded the AS + SDK bundle:
Assuming the defaults were left unchanged, you will find the SDK in
C:\Users\%USERNAME%\AppData\Local\Android
If it is missing at this location, one of the following reasons apply: you have chosen a custom location, you missed to install the SDK at all, or (unlikely) they have changed the default install location.
Note #1: the AppData
directory is hidden by default. If you have a standard Windows configuration, the C:\
path above should work "as is" (hit Win + R -> paste the path -> Enter).
Note #2: you will never find the SDK in the Android Studio directory unless you have explicitly put it in there. The reason is that those directories might be deleted after installing a newer version of Android Studio and you would have to re-install the SDK as well.
If you have retrieved the standalone SDK by some other means:
The SDK location may vary (from my experience), however (assuming the default configuration) you will most likely find it in C:\Program Files\Android
or C:\Users\%USERNAME%