Battery Life on Macbook Pro

I've got a 15" macbook pro, i5 2.4ghz which I've inherited. When I use it to watch video -- with the notebooks monitor turned off, video piped to an external monitor - the battery dies in 90 - 120min.

Is this normal? Does that mean its time to replace the battery? What sort of life should I be expecting?

You can check the battery's capacity to see if there's a problem with that. I use the free CoconutBattery. It will give you the percentage of the battery's full capacity (what it had when it shipped) that it has at this point.

It's possible that you may have some errant process running that are eating up your battery. Open Activity Monitor and click the CPU column head so that it sorts by CPU usage. Is there anything there that shouldn't be?