If a contact is manually added to a smart group (so has the status of 'Added') do they stay there for ever (until manually removed)? I am about 99.9% certain that this is the case. I've had some questions about contacts that shouldn't have been in a group (and so got an email in error) - and the reason is that they were manually added to the smart group. If true this makes manually adding contacts to a smart group a very bad idea - which I hadn't quite appreciated before. It makes the case for this feature to be optional so it could be turned off.
"You can override the criteria by manually adding or removing contacts from the group". So, unless the group is deleted or you remove them, my assumption is yes they will stay 'forever'.