The lifespan of a shower head depends on how bad the water is in your area and the quality of the shower head.
Showers heads have a lifespan of around 5 to 10 years but sometimes may last longer.
You can keep a shower head clean by removing the shower head every year or two and soaking it in some vinegar overnight.
Or soak the shower head in some CLR cleaner to remove the calcium deposits in the shower head and it should flow water like a new shower head.
I've always just used some vinegar in a coffee container and soaked the shower head in it overnight and it cleaned the calcium deposits out of it.