Help with calculations: length of time running on battery

I need help with this as I'm getting confused. Could somebody break down the calculations to figure:

How long will a 12V battery with an amp/hr rating of Y (assuming its based on 20hrs) maintain a steady watt load of X before the battery reaches 50% discharge?

Need to build a spreadsheet I can play around with where I can change the wattage and see how long I can go.

Thanks for the help!