Comment by Eiim

Flash memory relies on cells keeping charged, but the electrons can slowly leak and discharge the cells over time. It looks like the commonly claimed number is 10 years, but there's no clear answer. Hard drives also aren't great as a "set and forget" method. In either case you should refresh the data regularly (~yearly). Optical media is a great option for digital long-term storage, but paper is a very tried-and-true method, if stored in the right conditions.