How long would herpes be infectious if it got on your hair?

Seconds at the very: most. And, that would have to be under extreme laboratory conditions (right temperature, body salinity and ph). It goes when it dries. Hard enough to get it to live under optimal conditions. Furthermore, you probably can never get enough of it in hair to be infectious to people (maybe 1-2 viruses would grow in a dish). Doubt this is a risk to someone who didn't even have an immune system.