@Jing ***
In Bangkok you can apply when you have 45 days or less left on your current stamp
There is no down side to applying early as you don't lose any days applying early because your extension is added to the current expiration date.
you can apply right up until the day your current stamp runs out, BUT don't cut it too close because if you get out there and are missing a document or something you run the risk of overstaying until you sort it out.
Also IF you are using banked money method remember the seasoning of the funds is 2 months from the date you APPLY for your extension not from when your current stamp runs out