When Padre Pio was asked how long a particular soul would stay in Purgatory he replied "At least one hundred years. We must pray for the Souls in Purgatory. It is unbelievable, what they can do for our spiritual good, out of gratitude they have towards those on earth who remember to pray for them."
The length of time souls are detained in Purgatory depends on:
a) the number of their faults;
b) on the malice and deliberation with which these have been committed;
c) on the penance done, or not done, the satisfaction made, or not made for sins during life;
d) much, too, depend on the suffrages offered for them after death by friends and relatives still alive.
What can be safely said is that the time souls spend in Purgatory, as a rule, is very much longer than people commonly imagine.