The Progressive refers to a situation we are in the middle of. The Perfect Progressive looks back at that situation:
Similarly in the past and the future:
Without a duration expression (like for... or since...), the Perfect Progressive can be used for a situation which has ended before now:
Or before a past/future moment:
A situation that we're in the middle of is incomplete:
A situation which has ended might also be incomplete:
Because it's possible it ended before it was completed.
The Perfect Progressive can also look back at temporary situations:
If we use a duration expression (e.g. for three months), we can use the Perfect Progressive to talk about even longer periods of time:He's lived in Canada all his life.