Officially, they can stay there indefinitely. The biggest charger network in the area worked on an unlimited plan model for a long time, so it makes sense that they wouldn’t have a problem with fully charged cars hogging the network because it means fewer cars would charge and cut into their profits, however they don’t implement extra-fees-when-done-charging even for pay-as-you-go clients, which makes no sense to me.
In response, the city is by and large limiting charge times to 3h. Which is great if you have a fast charger, and very annoying if you have a plug-in hybrid.