If they let you when you want to.

One big thing that has prevented CNY from becoming a reserve currency is that China has explicitly said it wants to preserve its ability to heavily and suddenly restrict capital flows in and out of China. If all of a sudden you can't redeem CNY outside of China inside it that makes it a very poor storage of value.

You actually mean CNH as per https://news.ycombinator.com/item?id=47577249

CNY is internal to China! See https://simplefx.com/education/usdcnh-differences-between-ch...

To be clear cnh is a convention while cny is the iso standard.