I was looking into this the other week for an entirely unrelated project. There do seem to be well-established standards for UV exposure over time, though nail

I was looking into this the other week for an entirely unrelated project. There do seem to be well-established standards for UV exposure over time, though nailing it down in a layman-digestible (i.e. me) way was tough. IR-emitting security cameras, for example, will provide a diagram showing how long you can safely face the camera in various zones (it was usually measured in hours, so don't panic if you pass a camera on the way out of the office).