What is the validity period of a Non-B visa if a Work Permit is issued for only 6 months?

I know nothing about work permits as when I asked immigration on a 90 day check visit, all they said was " very difficult" do you mind if I ask where do you work in Thailand?

you have a 1-year extension of stay based on employment, and your work permit is active and not cancelled, yet. Just get your next 1-year EoS but show them the reduced time on your work contract. Immigration will issue the Extension according to the finishing date of your employment. Before day X, visit Labour Office, cancel the work permit, then you have two options: leave Thailand and let your extension become invalid (which I do not recommend !) or (better!) visit Immigration, cancel your stay permit, and if you need a little bit more time to pack up, ask them for the stamp "application of extension of stay denied, you have 7 days to leave the kingdom" (which would cost you an unneccesary 1900 Baht)

3 months+12

They will give you a work permit for as long as your contract.

I don't understand your post. The labor office follows the terms of your contract and the documents supporting your application for the work permit. So you have to check your documents and maybe speak to your employer.

I'm guessing from the way you've worded this that you're not on a visa at all but you are actually on an extension of stay. So to answer based on that, when you apply for the extension you actually sign a form stating that you will inform immigration if your circumstances change. So it doesn't matter how long of an extension they give you, when your work permit finishes you have to inform immigration and immediately leave the country. If you know it's going to happen you can cancel both in advance but you still need to leave by the day the cancellation comes into effect.