How long before my visa expires should I apply for a marriage extension in Thailand, and when does the 2-month seasoning period start?

At MOST immigration offices you can apply for a new yearly extension when you have 30 days or less left on your current stamp.

As far as the seasoning of funds using banked money method for a marriage extension you need the funds to be in the bank for 2 months from the date you APPLY for the new extension, not 2 months before the date your current extensions expires.