I don't warm up my R185 more than a minute or two. I start it up, run at half throttle for 30 seconds, and then increase to 'generator speed', about 1870 rpm. I don't load it fully until it's a bit warmed up, but I certainly don't run it for long until I put a load on it. I'd rather get it up to temp faster as it's more efficient and burns cleaner when it's at operating temperature.
I do have a 'genuine' ChangFa, though. Some of the ChangFa "style" engines are of pretty poor quality....
Chris