Harmonicas don't wear out but the reeds go out of tune and don't last forever. Reeds can last a few years, or can go flat in a few weeks. It all depends on how much you play, and how hard you play. Some people play very hard and loud. This reduces the life of the reeds. So, playing softly will prolong the life of your reeds.
Another thing to consider for maintenance is when you're done playing you should tap the harmonica on your palm (holes down). This will help remove excess saliva and moisture from the harmonica. After that, leave the harmonica in its case with the cover open. Let it dry for a while before storing it.
Here's some more advice: