How hot is too hot!

Hi, was wondering if anyone out there could help answer the question on how hot is too hot, my vespa T5 W5 172 is fitted with the SIP speedo and I'm constantly looking at the Cylinder temp and I would like help with what is the comfortable running temp and where it then becomes too hot and need to stop and let her cool down. Any help would be much appreciated.

cheers

Darren