Hercules became Alliant Powder in 1995 so it is at least 25 yrs old.  Is there a date on the can?  I have a 5lb plastic  jug of Alliant Unique someone gave me already opened quite a while ago I'm still using that is production dated Jun 29 1995 and appears to be still good.   I would think an unopened can stored properly would be good for quite a bit longer than that. But Hercules and 2400 had been around a long time before 1995 so it could be a lot older than that.