How much is too much?

Hi, after a hair transpltation (FUE or FUT) you have to protect your new hair from sun rays, but how much is too much?. I mean if I have been exposed to the sun how many minutes are too many?, what about cloudy days?, what about through a glass window? Is it so dangerous at 9:00 am as it is at 12:00 pm? how do I know if my grafts have been damaged because of sun exposure?