It's surprising how little information there is about washing papers. The reason may be the complexity of the issue. The paper, the fixer composition, the fixing time, the temperature, the washing apparatus, the pH, and a host of other things are all involved. Mees and James has a short section which is reasonably informative. Important factors they mention include: 1. Fix no longer than 10 minutes; 2. Keep the pH above the isoelectric point to retard absorption by the gelatine; 3. Use a high turbulence, minimum water washer that maintains fresh water across the print; 4. Ammonium thiosulfate fixers wash out faster than sodium thiosulfate; 5. Hardeners retard washing; 6. Salt treatments, like HCA, can reduce washing times; 7. Paper is a lot harder to wash than film.

Ref: C. E. K. Mees and T. H. James, "Modern Photographic Process" Macmillan, 1966. pp 403-405.