Our central a/c unit went on and the target temperarture was 73. I think it was 76 degrees at the time. It took about 50 minutes before it shut off. I never really had listened before but my brother thought that was a long time to run. I'm just not sure. It's a fairly new unit, only 2 years old.

Doesn't sound unreasonable. The thing is that once it got to temp how much did it cycle? When dropping 3 degrees and possible humidity it can take a little to get going. But it's the maintaining that can get expensive. So if the initial run was 50 minutes and then it cycles 10 minutes or so every hour your still looking good.

What was the temp outside? Have you cleaned or changed your air filter lately??