I use mine once every 4-6 months for 3hrs to sometimes 24+hours. After that, I put them in my closet and never touch them again. I have cases for all them. I have 4 1tb seagate go flex external hard drives. 1 of these hard drives is only a year old and the other 3 I just recently purchased. I also have a 320gb seagate external hard drives that is over 2 years old and is still in perfect condition. If I keep using my hard drives like this, how long should they last?
There is no real good answer to this question, in my opinion.
I have heard people say that hard drives typically last on average 4 years. When I heard this average mentioned, there was not specific statement as to what the usage pattern was, but I assume it was a fairly consistent/constant use. Assuming this number is accurate, then that likely means your drives could potentially last for a decade or more.
The problem is that there is just too many variables. It is possible that those drives could last for more than a decade. It is also possible that one (or even more than one) of them could die tomorrow.
So, to me, the best way to answer that question is to assume that ANY hard drive that you have could die 5 minutes from now. With that assumption in place, you then should evaluate what system you have in place to backup your critical data so that any one drive dying does not cause you to lose data. You can factor in that generally speaking the odds that two drives dying on the same day are rather low (except for situations like your house burns down and destroys both drives).
The point is that if you have the data on those drives in more than one "place" (i.e. have it on multiple drives), then one drive dying should not really impact you. And the more "places" (aka drives, discs, flash drives, etc) that you have your data, the lower your risk is to even multiple drive failure in a short period of time.