To help people who might search for answer to this same question, it is important to know what type of projects you are using in Studio.
Gradle
The default project type when creating new project, and the recommended one in general is Gradle.
For a new project called “Foo”, the structure under the main folder will be
Foo/
settings.gradle
Foo/
build.gradle
build/
Where the internal “Foo” folder is the main module (this structure allows you to create more modules later on in the same structure without changes).
In this setup, the location of the generated APK will be under
Foo/Foo/build/apk/...
Note that each module can generate its own output, so the true output is more
Foo/*/build/apk/...
EDIT
On the newest version of the Android Studio location path for generated output is
Foo/*/build/outputs/apk/...
IntelliJ
If you are a user of IntelliJ before switching to Studio, and are importing your IntelliJ project directly, then nothing changed. The location of the output will be the same under:
out/production/...
Note: this is will become deprecated sometimes around 1.0
Eclipse
If you are importing Android Eclipse project directly, do not do this!
As soon as you have dependencies in your project (jars or Library Projects), this will not work and your project will not be properly setup.
If you have no dependencies, then the apk would be under the same location as you’d find it in Eclipse:
bin/...
However I cannot stress enough the importance of not doing this.