When should I apply for a 30-day extension on my visa if it expires before my return from China?

You dont need to worry your visa validity will expire when you are away and you can just reenter on a 30 day visa exempt stamp if you are only going to be back here until your condo lease is up. You dont need to do anything

Um... doesn't make sense to get a 30 day extension and purchase a re-entry if you can get a visa exempt when you return. What is your nationality?

Be careful with tourist visas when you are living here...safe travels.

You can apply for an Extension of Stay in almost every Immigration Office from 30 days before till the last day of your Admitted to Stay date. The Extension is added to your current stay, so it does not matter if you go 29 days before of on the last day. If you visa is valid until/enter before date Jan 8th, you can not use this visa after this date. If you want to travel and still have a active Admitted to Stay period in your passport you can buy a re-entry permit, if you come back before that stamped date in your passport you will get the same day stamped on entry.

If you are coming to the end of 6 months you can leave and come back into Thailand before your visa expires. You will then be stamped back in for 60 days which you can then extend for 30 giving you 9 months in Thailand. This is how our UK METV works. Not sure if it is the same for US.