where are custom extensions installed in visual studio?

Extensions (if deployed as VSIX) will be installed to the user´s profile; each extension will be installed into a folder with a random name, for instance:

%LocalAppData%\Microsoft\VisualStudio\12.0\Extensions\s5lxc0ne.1kp

If you want to obtain the package installation path at runtime, you can obtain that information from the assembly that defines the Package class.

static string GetAssemblyLocalPathFrom(Type type)
{
    string codebase = type.Assembly.CodeBase;
    var uri = new Uri(codebase, UriKind.Absolute);
    return uri.LocalPath;
}

...

string installationPath = GetAssemblyLocalPathFrom(typeof(MyPackage));

1- Find your package... Let's say your package is MyExtensionPackage.

public sealed class MyExtensionPackage : Package
{
     //...
}

public static string GetExtensionInstallationDirectoryOrNull()
{
    try
    {
        var uri = new Uri(typeof(MyExtensionPackage).Assembly.CodeBase, UriKind.Absolute);
        return Path.GetDirectoryName(uri.LocalPath);
    }
    catch 
    {
        return null;
    }
}