The worst thing young adults can do is go to graduate school because they aren't sure what else to do or they can't find jobs. Grad school, after all, is often an extremely long commitment. A Ph.D., for example, can take six years.
The average lifespan for an air conditioner is 10 to 15 years. Some air conditioners last up to 20 years. As interesting as these data points may be, they don't tell you the whole story. The issue is that a variety of factors affect the lifespan of an air conditioner.
How long should it take to cool one degree. I have purchased a 3 ton carrier central air unit (outside and inside) on a 10 year old 1350 sq ft house with added insulation in the attic, they did not change the furnace or the blower. They came out and gave me a quate and I told them I want the air to come down when I turn it down.