Power consumption
The Arduino boards use a fair bit of power compared to other embedded systems with similar functionality.
There are two main factors:
The NCP1117 (datasheet) 5V linear regulator in the Arduino UNO
R3 (schematic) has a quiescent current of around 6mA.
The ATMega328P (datasheet) draws around 5mA @ 8MHz and 5V, and probably more than double that at 16MHz.
Furthermore, the ATMega16U2 used for USB communications also draws some power, along with the LEDs and other peripherals. In your circuit, the LCD backlight is probably drawing 4mA as well.
A duracell 9V battery (datasheet) drops from 9V to 7V in around 7.5 hours with a 50mA current draw. Therefore, a rough guess is that your circuit draws around 25mA, which sounds about right based on the description of your circuit.
Note, alkaline battery life is non-linear with respect to current. For very small currents (<1mA) the life of an Alkaline approaches that of a lithium battery.
Getting current down
Here are some tips to get the current consumption down:
Regulator: Replace the regulator with one with a low quiescent current, or better yet, a buck converter. Sparkfun etc have buck converters that take the battery as input, then connect directly to 5V and GND, bypassing VIN and the regulator. Alternatively, you could buy a rechargeable lithium battery with charging shield. This advantage of this is not having to buy new batteries, and for a tiny bit bigger than a 9V battery the lithium one has much greater capacity.
ATMega328P: Rather than using delay for timing and spinning in loop endlessly waiting for something to happen, re-write your code so that it goes to sleep in between sensor reads, etc. There are a few low power libraries out there that use the watchdog timer for periodic wakeup from sleep which are handy. You can get the current consumption of the ATMega328P below 0.1mA during sleep.
LCD: Turn off the backlight, or even the entire LCD. Add a button to the design that the user can push to activate the LCD and have it turn off after a set amount of inactivity.
Peripherals: Most peripheral chips also have a sleep mode that drastically reduces their power consumption. Remove power LEDs and other indicators that are not necessary.
Summary
With appropriate power supply and coding you can get a reasonable amount of life out of a battery. Using the above principles, I have made an Arduino derivative board that measures a few sensors and stores the readings to an SD card every half a second. It can last for about 4 months on 2 AA batteries, so it is definetly possible to have low power and remain in the Arduino ecosystem.
