While son was in HS gloves seemed to last for 2-3 years, in college they lasted about 1 1/2 to 2 years, however he was using 2 different gloves in college so the average life was probebely a season catching guys throwing 83-97.
He always keep the gloves cleaned and oiled, you never saw puffs of dirt coming out of the glove while he was catching. He never threw the glove, jammed it into a bag, left it on wet grass, dirt etc or touched the ground with it unless blocking. Dirt is like sandpaper to the glove. Bottom line if the glove is becoming floppy or torn, its time to retire it.
Professionally its hard to say as I think he now changes gloves before they become "less useful" as the only issue is breaking in a new glove rather then cost. At the begining of the last two seasons he started with two new gloves in addition to his various hold overs. I might also add the gloves he now gets are made of higher quality leather then those you would typically find in a good sporting goods store.