I'm with Donald, 45 to 60 minutes in an archival washer is my normal time. Using the method you describe with prints image to image and back to back does not guarantee that all fixer will be removed, there is a danger of uneven washing. Before I used an archival washer I used a Kodak Tray Syphon on a tray one size larger than the largest print being washed but put only two in the tray and regularly turned them over. It was tedious but I never had a problem with staining etc.,