well, the huge rift valley i live in has about 3000 foot deep sediment (rift valleys are not common features in the U.S. but this is one) because as the fault block mountains pulled apart, the gap was filled in with sediment from the erosion of the uplifting mountains. (the sediment accumulated because there is no outlet to the valley and thus no transport out.)
So that means the aquifer is relatively level in the sediment zone (the sediment zone, because of being rich and level, is where the agriculture is) and it has been drawing down pretty uniformly. They use a few feet per year, depending on which crops are being irrigated.
There are a few bubbles in the level of the water where the water is shallower, but they are mostly hot springs... there is so much hot water here that i know several ranchers who irrigate with hot water, after letting it sit in impoundments to cool. The geothermal resource is mostly ignored here... the hospital and the school is heated with it, but curiously, one can buy land with hot water on it relatively inexpensively. The U.S. Geological Survey has drilled several geothermal test wells, and where they drilled on selected sites, most all of them came up as power generation grade... the problem is there are no transmission lines capable of carrying the power out, which would really be spendy to construct.. Perhaps some day hydrogen electrolysis will make it viable to use the energy efficiently, or if power demand got high enough, someone would invest in transmission lines out.
To answer your question more directly, there is likely 2000 feet or so of fossil water. The problem is that the deeper one goes, the spendier it gets. But the 1/2 cent per kilowatt hour makes the break even point quite a bit deeper than where they are now. Dry holes are very rare, unless one is drilling near the edge of the valley and hit a bedrock seam.
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