They're ending next month 20. How far in advance should I go?

and: any advantage by going before the expiration date? disadvantages if going ahead of expiration date?

You can apply up to 30 days before expiry of your current permission to stay (45 days at some offices).

There are no disadvantages to applying early. The only advantage of an early application is that you have time to rectify any issues that are identified by your immigration office.

Regardless of when you apply, the one-year extension is added to the end of your current permission to stay.

Whenever I do my yearly extensions, I go about 2 weeks prior, that way I still have time to fix any problems if they arise without feeling rushed.  You never want to wait to the last 1-2 days, also check to see if there are any holidays coming up where banks or government offices will be closed.