About all we can say is it’s finite, and time there doesn’t necessarily exist like time here.
Likewise, there are different levels of purgatory. If you just need a little brushing-off, you go to a higher level for a very brief period. If you have some serious scrubbing to do, you go to a lower level, stay there for as long as you need to rid your soul of its remaining impurities, and work your way up. It varies. A consecrated religious will suffer more than an ordinary person. Someone who was deeply attached to their sin will suffer more than someone who wasn’t.
But to a soul in Purgatory, according to what I’ve read, any amount of time is too long, because no one goes to Purgatory without first seeing God as he is-- in other words, getting a taste of the Beatific Vision that everyone in heaven experiences eternally. To have seen that, and then be deprived of it for a period, is a very difficult thing to suffer, but no matter how much suffering a soul experiences, they still have the joy of knowing they will one day experience that forever.
Who was it who said, “A moment in Purgatory is more painful than a lifetime on earth?”