I'm happy to leave it plugged in 24/7, but is there any PC programs that can take the data from the serial monitor and put it onto a excel document? (bonus points if it can average the readouts over a 12 hour period).
I use this http://www.parallax.com/tabid/393/default.aspx
It will give you a real-time graph if you want it.
Another alternative is RealTerm, which accumulates data that may then be incorporated into Excel.
It is also simple to log to an SD card, making a new CSV file every day which can then be used in Excel.
If your company is sufficiently paranoid, you can send the data to cosm and they can check up on you whenever they like.
And any of the above can include any averaging you can think of.
You could do what I do, which is all of the above, and an LCD display too. If you do, you will probably need a Mega, if you don't already have one.