Bit persistence on a Hard Disk Drive
The magnetic domains of the HDD surface are designed by the manufacturer to hold their state for a long time. That said there is always a leveling effect where adjacent domains of opposite polarity can tend to pull the both of them to a similar level. The advanced techniques used on today's HDDs really try hard to minimize this effect whilst at the same time making the read back circuits sensitive enough to be able to still see the polarity reversals when the domain to domain changes are much less than when originally magnetized. 

Additional techniques such as embedding additional Reed Soloman type error recovery data bits into the recorded tracks can help to allow data recovery when some domains have degraded too far. 

I am sure that manufacturers of HDDs try to assure that there products will retain data for a long long time. They may even specify an MTBF number to a certain level of errors. But remember that is a mean specification and some failures could happen before that time and some after that time. 

At the end of it all it is pretty certain that the archival storage of data on HDDs is best done using a combination of redundant storage on similar media, redundant storage on alternate media type and periodic copying of data to ensure fresh recordings. This also means cycling out older HDDs as they have been re-used a certain number of times.