How much water do aquifers contain? How much does it take to deplete them?
It depends on the size of the aquifer and the amount of precipitation falling on the exposed areas of the aquifer. These have been used to provide water form many major cities.
The Trafalgar square fountains were initially powered by artesian pressure (as London is below the altitude of the North downs / Chilterns). Eventully too much water was extracted leading to salt water incursions in places. This aquifer is still in use but backed up by a number of surface reservoirs surrounding the London area.

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The Report of the Royal Commission on Water Supply (1869) shows that, by 1869, a number of large conurbations were supplied by groundwater. Nottingham and parts of Liverpool and Birkenhead relied on wells in the Permo-Triassic sandstones, Sunderland and South Shields on the Permian Magnesian Limestone and Croydon and parts of south east London on the Chalk. Government offices around Westminster together with the fountains in Trafalgar Square were supplied by wells sunk to the Chalk in 1844 (Amos, 1860).
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More from here http://www.groundwateruk.org/British-Hydrogeology-a-brief-history.aspx

More detail on the London aquifer:
 https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/735451/2018_Final.pdf