We know it is temporary, and the Catholic belief is that the detour varies according to the individual burden. We should however be careful when trying to quantify the length of the stay. The popular notions that a certain prayer releases 1000 souls from purgatory right away is something that we are nto required to believe, -- these are private revelations that do no harm if you believe them, but there is no harm is being skeptical about them. The important thing in the Catholic teaching is that all souls in Purgatory are on their way to Heaven.
The Orthodox belief, as I just learned in this thread, is somewhat different.
I did think that the Catholic teaching is more trying to understand the two realities that God would like us to be pure and sinless in Heaven and are we like this -- what about unbaptised babies etc. -- what happens to them? The early Church probably shrugged and said, only God knows (which is true) and we understand them to be in Limbo (in a state/place unknown to us or not understandable by us) -- we just don't know.