There is a date printed on the bottom of each can. This date indicates when the can was filled with gas, not any kind of expiration date.  The shelf life is 10 years when properly stored.  If you keep a duster or horn for a couple of years, we recommend periodically inspecting the can for signs of rust, misuse, compromise or damage.
We suggest that horns be replaced every 10 years as part of a preventative maintenance program for safety equipment.  (have link to Safety Horn Inspection recommendations here) As with all safety equipment, an inspection schedule should be implemented as part of a normal routine (such as testing horns the same time as your annual fire extinguisher inspection).