I have been doing direct unfiltered sunlight only with my male panther Ryker due to eye sensitivity to artificial lights. He goes outside and I sit with him at least an hour, often a bit longer. I am making an outdoor cage that is friendly for his partial blindness at the moment so I can safely leave him outdoors and not worry about it. I do 1 hour of sunlight a day and then no uvb at all for the rest of the day. He does fine and has shown improvements in many ways that I won't go into here because this is your thread.
As far as how long they can safely go without UVB if they have no sunlight at all and have no artificial uvb that day (because ambient temps and weather do not always play in our favor) if you do 2-3 hours a day at peak sun hours for your part of the world. And then are forced to go 2-3 days without uvb in any form, in my opinion and probably many older keepers who had to rely on natural sunlight only, this should be safe.