5 minute battery life
While I can't explain the rapid loss in battery life, the best way to determine the battery health would be through the system profiler. There are instructions on how to check the battery cycle count on a Mac here: http://support.apple.com/kb/HT1519

Along with benchmarks for healthy cycle counts.

For what it's worth, my 15" MacBook Pro needed a replacement battery a few months ago. Although the decline in battery life was a little bit more prolonged.