It's ridiculously difficult to gague exactly how long a digital camera's battery(ies) will last. The use of such power-draining devices (flach, 2.0" LCD screens, etc) bring in too many variables. However, as the former lead of the Digital Imagine department of a local technology store, I can make a reccomendation.
Go to your local RadioShack, CompUSA or Best Buy and purchase a $30 - $40 pack of rechargable batteries (if the camera is capable of using AA or AAA). It has a charge and 4 batteries. This way, you can charge them fully, and even when one pair runs out, you can have a second pair with you as a backup.
Like I said, however, with a Li-Ion battery, there's not a highly accurate way to measure the life of the battery without manually draining it yourself and recording the time it took for it to die.
$0.02