2000 mercury sable problem starting up

Hi, I'm new to this forum and have little knowledge of cars in general. I'm hoping someone can help me out.

When I turn my key on my 2000 mercury sable when it was idle for more than a few hours, it would rev but not start. I would turn off and start up again. The car then starts up immediately. On one occasion I needed to turn off and turn on a 3rd time before it started. Once the car is running and I turn it off, it starts back up without a problem. There's only a problem when the car is idle for more than a few hours.

Around January I decided to have a local oil change place change my battery. Around May, sometimes I would turn the key and hold for a long time before it finally started while other times it started up just fine. Then one day the car would rev but not start anymore. It came to a point where I turn the key and all the the car did was click (no more reving). My normal mechanic told me the battery was dead and replaced it. When I went back to the mechanic with my present problem, the car started up fine.

I'm worried that something's draining my battery and in a few months I'll have to replace my battery yet again. It's also possible the oil change place sold me a bad battery.

You might want to have your alternator tested. A lot of times when people think their battery is bad it turns out to be the alternator. I know Auto Zone can test them but I don't know if they can test them while it's still attached to the engine.