How long can the EU stay permanently and return to China?

The longest time you should leave is 12 months.

Non-European immigrants with permanent EU status who return to China or go to other non-EU countries, as long as they return to EU member countries within 12 months, their permanent status will not be revoked, otherwise they will lose their permanent status.

If non-European immigrants leave Italy for other EU countries after obtaining EU permanent resident status in Italy, as long as they leave Italy for more than 6 years at a time, they will lose their permanent residence permit in Italy.