No harm is done to the devices if juice is left inside. I've left juice in them for quite a few days with no problems. This is one of the reasons that I prefer the smaller devices like the mini T3s or other small clearo's. I like switching flavors throughout the day so I'll fill about three or four with my favorites. No 1 flavor will sit for longer than one week. If it's less than half full and I don't think I'll come back to it, I'll dump it out.