It wouldn't be an upper bound if energy were free, but it isn't. https://worldpopulationreview.com/country-rankings/cost-of-e... lists China as US$0.08/kWh, but that's the residential price; industry pays a bit more. But it's a countrywide average.

Intensive investigations of Chinese "subsidies" from the US Department of Commerce have come up with embarrassingly little.

The current €0.100/Wp price level (12¢/Wp) combined with the US$0.08/kWh given above does give us an upper bound of 1500 hours of full sun for the energy payback time. At a Californian 29% capacity factor, that's 7 months of operation.

But in fact not 100% of the cost of the solar module is the cost of energy; there are some labor and raw materials costs as well, as well as capital costs and, astoundingly, some profit. So 3 months is probably a better estimate.