Either China will catch up on this or that particular technology will become obsolete. But it is certain that they won't stay behind forever (measured in a small number of decades at most).
What is far less certain is what ASML will be able to do at that time, i.e. if they will be able to progress significantly over the state-of-the-art of today, or they will reach a plateau.
Besides China, there is a renewed effort in Japan to become competitive again, so ASML may face in the future both Chinese and Japanese competitors.
You might place an upper limit using history but in this case I'd guess that limit would end up being much larger than the present semiconductor industry itself might last.
There is a level of arrogance in the West that China does cheap but simple/low quality whereas this is only a stepping stone along the way. German car manufacturers went into China during the 90s with that mindset, and expecting it was forever, well they don't think that anymore...