Convert 9v of battery into 5v at 3.5watt
The data sheet available on this page won't tell you outright but it'll hint at very bad news.

It shows that at a power draw of 0.25W the service life would be 14 hours. However at 0.5W the life falls to - not 7 hours, but 3.5 hours. This shows that pushing the battery beyond its limits results in useful life much shorter than a simple amp-hour calculation would suggest.

Double the power 3 more times and if the same pattern applies, the projected life would be more like 4 minutes than the 36 minutes that Samuel suggested.

Find a battery with your current requirements at least on its design spec. Even four AA cells would a closer match for your needs.

Alternative approach : ditch the R-Pi and use a lower power target processor. This depends on your computational needs, but there are low cost ARM processors like the TI Stellaris and others, with maybe 10% of the processor speed but capable of running at a few ma and sleeping at a few microamps for maybe a year on that battery.