When shall I get iOS 16 update for my iPad 8th generation?

Unlike iOS16 for iPhone, released 12th September (iOS 16.0.1 subsequently released 16th September), iPadOS16 is not due for release until sometime in October. 

However, iPadOS 15.7 was also released 12th September - incorporating essential security updates.

About the security content of iOS 15.7 and iPadOS 15.7 - Apple Support

While iOS16 is available for iPhone, iPhone owners have the option to just install iOS15.7 at this time.

When iPadOS16 is released, it will be available for these models of iPad:

• iPad Pro (all models)
• iPad Air (3rd generation and later)
• iPad (5th generation and later)
• iPad mini (5th generation and later)

More information about the forthcoming iPadOS16 can be found here:

iPadOS 16 Preview - Apple

Per Apple.

iPadOS 16 coming sometime in October 2022.

Tap this link below.

iPadOS 16 Preview - Apple

When it comes out next month, if you have a compatible device.

You can update to it this or next month when it release (or try beta), however there are limitations on some features.