It varies widely by soil type, weather, etc. Someone who has a similar pond in TX might be able to ballpark it for you, but the best way to do it is to measure it yourself.
Set a stake in the bank at the current level. Wait a few days during which there's no flow of water to the pond, then go measure the vertical drop between the two stakes. Assume no significant decrease in the surface area of the pond and just calculate the volume loss. If you get these two measurements I'll help you with the math.
