Arduino battery life

I am considering using the arduino for some projects but I was wondering how the battery life for just something simple like an arduino, a switch and a sensor is. Assuming I use tiny lithium batteries.

It's impossible to say without details for the battery and the load(s), but a previous thread at http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1243213127/ should help you extend the battery life, no matter what the specific values are.