1 For surface waters, between 20 and 40 mm (upstream the strainer)  For municipal wastewater: for raw water, from 15 to 30 mm (but upstream from a straining and/ or lamellae settling process, fine screening is necessary); for sludge (if necessary), 10 mm or less.
Other studies. According to some studies, 18-year-old males have a refractory period of about 15 minutes, while those in their 70s take about 20 hours, with the average for all men being about a half-hour. Although rarer, some males exhibit no refractory period or a refractory period lasting less than 10 seconds.
Any interest in sex you had appears to be gone and some unfortunate men want nothing more than to sleep, even as their partners sulk. The length of this refractory period varies from man to man, with some men taking 15 minutes while others may take as long as 24 hours. This is one of the reasons why if you jerk off in the morning, you have no interest in your partner later that night.