what is the max duration of layover in Guangzhou (CAN) ?

Here is the "right answer" according to Timatic database as of today. In summary, you do not need a transit G type for an 8 hour connection in Guangzhou.

National Nepal (NP) /Residence Canada (CA)
Transit China (People's Rep.) (CN)
Destination Canada (CA)
China (People's Rep.) (CN) TWOV (Transit Without Visa): Visa required, except for Holders of confirmed onward air, cruise or train tickets for a max. transit time of 24 hours. Transit incl. multiple stops within China (People's Rep.), with a total transit time of max. 24 hours, is permitted. They must travel to a third country. Transit without visa (TWOV) is not possible at Fuzhou (FOC), Mudanjiang (MDG), Shenzhen (SZX) and Yanji (YNJ). This does not apply at Urumqi (URC).

Hi Leo, would you be so kind to check for me if a Pakistani national with residence in Singapore needs a transit G for the route SIN-CAN-YYZ (transit time is 3.5 hours, visa for Canada is on hand). Your help would be highly appreciated. Thank you, Henrike

According to Timatic, Pakistanis do not need a visa except when transiting through Xiamen or Urumqi. However, certain Chinese airlines do not follow Timatic rules and demand travelers from certain countries to hold a transit visa. You should contact Guangzhou immigration office during business hours as well as your airline.

Great. Thank you so much for the info!