How long before my extension of stay based on marriage expires can I apply for another year in Bangkok?

You can apply up until the last day but not advisable to leave it so late.

I give it 30 days before end date. Works for me in and out 20 minute's. Extension by marriage they always need more time than a straight up retirement.

You don't need any time left. It's not like when you're switching from another visa type to non-O.

You can wait until the last day of your stamp to apply. But definitely don't do that because it's highly possible you are missing something and need extra time to finish gathering documents etc.

If you are asking about a 1 year extension then you can apply 45 days early in Bangkok.