How often does "bit rot" occur?
I tell people (nobody listens) that ALL hard drives will crash, and that this occurs sometime between 10 years and 10 minutes from new. 

I've not yet seen one reach 10 years of continual use - but I've met a few 10 minutes ones. 

CDs that I've checked that I wrote 15+ years ago are generally still OK when I try them - casual check only. Data on them is either now irrelevant or redundantly stored elsewhere. 

If you want a 50 + year archive (I do) then you MUST work at it with redundant backups, off site storage and more.

Increased temperature kills hard drives. 

Brand matters. Google know but won't tell. 

Vibration doesn't help.

Some CD/DVD material is better than others and the top manufacturers have archival material that they claim will have 100 year life if stored as recommended. Don't trust to this if you care. 

Powered on drives seem to last as well as those powered on less often BUT it seems likely that if you have a drive which is ONLY used for backup that it will last well if powered up only when used BUT store at good temperature etc when off. BUT if it uses Aluminum wet electrolytics they will die FASTER when the drive is unpowered!!! If you wanted really long hard drive life with occasional use you'd have to work at it and still use several redundant copies. 

50 year backup has to survive standard electrical hazards and disk death + fire, theft, earthquake and operator death. 

Online storage can help as it puts the burden on those who SHOULD know BUT do not trust it. Do not use Megaupload :-). Multiple independent online storage sites can help. Ensuring true independence may be tricky. As google moves to own, er, store all the world's data you may find that vendors A B and C also use Google behind the scenes. 

I have almost 1 million photos stored. I wonder if my great grandchildren will get to see any of them ? :-). 119,204 / 90.55 GB  of them here That's paid up 10years ahead BUT no guarantee it will be there 10 years from now.