How long does the 800,000 baht need to be untouched for my NON-Immigrant O Visa Renewal before the expiration date?

The rule about seasoning is not when your current extension expires but from the day you apply for that extension.

At most offices in the country you can apply for a new extension when you have 30 days or less left on your current stamp, while at some offices, Bangkok, Chiang mai, Phuket and a few others you can apply for it when you have 45 days or less left on your current stamp

The money needs to be seasoned in a Thai bank account in your name only for 2 months before the date you apply for the extension.

So decide when you're going to apply and do the math.:)