There are a lot of variables affecting readable lifetime, but DVDs do degrade rather more quickly than one would like. It's a good idea to refresh media every few years.
Well yes. From about 2002-2004 I used DVDs for data backup. In 2006 I copied all of them to HD. As long as the files are in containers with checksums, data can safely be copied indefinitely without loss. It is not a problem for me to do so every few years, but a major issue with organizations that archive data.
