Purgation could be instantaneous upon death, but there is nothing precluding the possibility of purgatory having duration. Eternity is an all encompassing event that includes the past, present, and future in some kind of panoramic and seemless experience that never ends. This does not suggest that there is no past, present, or future. It merely means that these things are experienced outside of our earthly experience of time which is a construct necessary to the physical world.
If you deny the possiblity of purgatory having duration based on your scenario, then you are compelled to reject other things pertaining to the after life. For example:
–it would be necessary to believe that the moment a person dies they would simultanesouly experience the resurrection of the body.
–it would be necessary to believe that the martyrs mentioned in Rev 6:9-10 have already witnessed the Lord judging and avenging their blood on those who dwell upon the earth. Scripture, however, indicates that they are waiting for this to happen.
–you would have to believe that the heavenly liturgy, described in Revelation chapter 4 and 5 which describes a sequence of activities by the twenty four elders, could not happen as sequential events.
The examples could be multiplied but those given should suffice. Clearly, we do not understand or comprehend what things will be like in eternity. We will be changed but we can only speculate how all of this is accomplished.