Individuals should focus on both the duration and frequency of handwashing to help prevent the spread of COVID-19.
Studies show that within one hour, hands typically become as contaminated as they were prior to washing them. Given that COVID-19 can transmit via surfaces, as well as respiratory droplets, it is important that hands are washed at additional and different times to what would normally be recommended for the control of diarrhoeal diseases. Information about when hands should be washed is available here.
Recommendations about handwashing duration will vary depending on whether guidance is aimed at the general public or health care workers. Unsurprisingly, it is particularly important that health care workers and patients or visitors to health facilities wash their hands more thoroughly and frequently than the general population.
However, there are also different recommendations about how long hands should be washed for within domestic or workplace settings.The WHO recommends that the handwashing process (from wetting hands to hand drying) should take between 40-60 seconds. They also suggest that hand cleaning with alcohol-based hand rub should take 20-30 seconds. Meanwhile, the Centres for Disease Control and Prevention (CDC), recommends hands should be washed for 20-30 seconds.
In reality we know that most people wash their hands for less than 10 seconds. These findings come from studies conducted in high income settings where soap and piped water are readily available (study 1, study 2, study 3), so in water scarce areas or low and middle income countries it may be much less.
During the COVID-19 pandemic, the most common guideline adopted by national governments has been handwashing for at least 20 seconds. However, the evidence to support this recommendation remains weak. In particular there are no studies showing how long it takes to kill or remove SARS-CoV-2 from hands as these would be dangerous to undertake. Studies on other pathogens indicate that hands will become cleaner the longer they are washed, however the process will reach a point of diminishing returns (where lots of effort is required for fairly minimal additional pathogen removal). The table below shows typical reductions of common pathogens according to different durations of handwashing.
Source: Bloomfield et al.
Overall, it's important to promote handwashing frequently to reduce COVID-19 transmission and the transmission of other pathogens. Recommending long durations of soap lathering may make it a less feasible behaviour in areas with water or soap scarcity, so the recommendation is to target a duration that is feasible given your context. This, in addition to frequent handwashing, will contribute to preventing the spread of COVID-19.
Below is a guide from the WHO on handwashing in health care facilities. Although it is aimed towards health care workers, it demonstrates different handwashing techniques used to ensure the entire hand surface is covered during handwashing.