I reckon the Chinese Air Force could operate airborne tankers some 300km off the Taiwan coastline, which is only 600km from the Chinese coastline.J-20 would not be regularly operating 1500 km away from china, the same way Superhornets would not be operating at over 1000 km from the carriers, most of the time. Sure, occasinally, some J20s may get to perform an operation 1500 km away from China, after refueling. But it wouldn't be patrol, it'd be some escort mission or strike mission. As a rule, though, they'd stick closer to China, as it's very unlikely they have such range. and why are we even talking about J20 range when there's no way for us to know such figures?
Remember there is likely to be a SAG or Carrier Group in the same area.
So establishing a continuous CAP (with J-20/J-15/J-11) at 1000-1500km from the Chinese coastline would be feasible.
Note that 10 years ago, doctrine looked at F-22s operating from Guam to Taiwan.
That is 2800km outbound and would require 2 air refuelings. Then the same again inbound.
So Chinese J-20s operating 1500km from mainland China would be following a similar operating profile
But 1500km is only half the distance that the F-22 was planning on.
Also remember that the J-20 and F-22 are roughly the same size, but by most accounts, the J-20 has more fuel capacity and therefore range.