My previous Sr tech used to take about 2 hours diluting Conc Sulphuric acid, by adding the acid to water at a very slow rate whilst stirring and standing the beaker in a cold water bath. Simple and fast and safe method: Crush enough ice to fill a 2 litre beaker to three quarters full. Measure out specified amount of acid in a measuring cylinder. Pour acid over ice, then leave to melt. Fill your storage container (in my case a 10 litre aspirator) up to 7 litres with water. Add contents of beaker through a funnel.