24 hours urine collection means the whole urine lot that you make over 24 hours. The volume depends upon the amount of fluids that you are used to taking, and may vary from 1 to 4 litres. Best way for the collection is flush away the first urine sample of the morning, and then keep on collecting in the container all that you pass till next morning, including the first urine lot that you pass on the next day. Ideal 24 hours urine output would be approximately 2 to 2.5 litres.