However there are definitely time limits. You would find it hard to be enrolled on a doctoral programme for 6 years or more especially if you aren't about to imminently graduate, and this is likely the main motivation behind the advice given the proposed 3 year break in China.
OT: How much? Undergraduates too?
Bachelors and masters pay fees according to: https://www.epfl.ch/education/studies/en/rules-and-procedure... or https://ethz.ch/en/studies/financial/tuition-fees.html
Tuition is definitely not free, but also not extortionate.
AFAIK, as long as you are registered as a PhD student, you have to pay the fees.
That's easy to pay if you are an ETH employee with a normal salary.
It might not be that easy to pay if you are in Wuhan university, with a Wuhan PhD student "salary".