In normal operation, there’s no need to drain the tank or take it out of the printer for storage. The orange cover is very effective at blocking the UV light and the resin has been designed to have a good shelf life. I top off the tank as I use up resin with prints, but any unused resin just stays in the tank for the next print.
I’m not sure that I’ve seen any specific numbers on how long you could leave it under the enclosure, but certainly multiple weeks, and probably several months.
One thing to keep in mind, of course, is that you probably shouldn’t open the orange cover up if the tank would be in full sunlight or some other strong UV light source. At the very least, you’d want to close the cover ASAP to avoid unwanted curing.