Hydrotest or what is formally called a hydrostatic test is a measure to examine the non-existence of a leakage.
It is usually done on a vessel or tank that is manufactured to a standard.
According to ASME VIII Div. 1, the pressure required for standard hydrostatic test is minimum 1.3 times the maximum allowable working pressure (MAWP).
Specifically, this 1.3 number is subjected to every point in the vessel not just a point. Indeed, the test should be conducted at 1.3 to the highest pressure point.
Duration typically starts from 30 minutes, 1 hour to 24 hours. If the pressure cannot hold the pressure, water leakage will show at 30 minutes and does not require 24 hours.
There is, however, a disadvantage of testing overnight. When the sun sets, temperature starts dropping and water volume decreases over time. The lack of water quantity in the vessel will give erroneous lower reading in the pressure gauge as if there was a leak; although the shop floor is sand dry.