What is a typical upper frequency limit for low SWR operation of 50-ohm BNC connectors?

Explication

The 50-ohm BNC maintains a controlled impedance and low SWR up to roughly 4 GHz, above which its size and construction stop being a good match. Fifty and five hundred megahertz badly understate its capability, while 40 GHz is the domain of much smaller precision connectors.