I got my 2 month tourist visa with a one month extension on November 19th last month and im wondering when I have to go get the extension by. I have heard people say you need three valid weeks left in Thailand to apply for one. Any feedback?

The 30 day extension of stay on a tourist visa can be applied for up until the end of your permitted to stay - normally you should do this about a week early as you lose no days by doing so.

If you are talking about a conversion to a retirement non immigrant visa/extension of stay they yes, you need to do that with at lerast 21 days remaining.