MouseSavers has tracked historic discounts which you can check here (
https://www.mousesavers.com/historical-information-on-walt-disney-world-resort-discounts/). December is usually a max of 10 to 20% and the percentage will depend on the resort. With COVID, however, I would not bank on getting a discount. I could see there being no discounts if things are back to normal-ish and there is pent-up demand. There is also a chance that big discounts will be available due to COVID and the financial situation if attendance has not bounced back.