Let's suppose that I filed a non-provisional patent application in the USA today, how much time do I have to file the patent in China before it is no longer patentable?
Same for the EU.
The central consideration is that most of the world requires absolute novelty of an invention as of the day of filing. The U.S. is an exception with a sort-of one year grace period.
As long as your invention is not made public you can file in China and in Europe. The default is that your US application will be published by the USPTO 18 months after the earliest claimed filing date. If your non-provisional claims the benefit of a provisional then it’s the filing date of the provisional that starts the clock.
If you keep your invention secret you could wait to file until just before the 18 month point but there is no guarantee that it will not be published before some exact date so this is dangerous.
The most important international agreement relative to this question is the Paris Convention. It says that filing in any “Paris” country counts as filing date everywhere else - for one year. Assuming you file in the US before disclosure you have essentially filed everywhere else before disclosure (assuming you actually file with the year). If you file in China or Europe a year later then it’s ok that your invention became know publicly.
This date also counts in terms of what prior art is allowed to be used against your application.
There are other international agreements that allow you to stretch that year. A PCT application is a bundle of 144+ applications in as many places. If filed during the year it holds your place in all those places until its deadlines come in to play.
If you filed in the US with a non-publication request you could keep your invention secret, wait until just before paying the issue fee and file wherever you like. Very importantly you need to rescind the non-publication request. I would do that before paying the issue fee.
This leaves you open to any prior art, published by anyone else anywhere in the world, up to the day you file.