In South Korea, schools were closed on Feb 23, and on March 10, their total cases started to level off. That's 16 days of social distancing, if we take school closure as a marker of it. Presumably some of that time was because people who were exposed beforehand had to go through the incubation period. And there was also some transmission within households and between those considered essential.

Italy closed schools on March 4, so not yet 16 days, and cases there are still increasing at more or less the same rate. But the next few days will be telling. ETA: 5 days later, Italy's daily case numbers and daily death numbers are still increasing. There are anecdotal reports that both are probably being undercounted, as the system there is overwhelmed.

ETA: China has been in lockdown for almost 2 months, and is now starting to get back to normal.