Full backups must be done within a 24 hour period. Full backups are done one day per week. There are no network problems durring backups. There are no AFS problems durring backups. One tape takes 3 hours to write.
All tapes have no errors and are the correct length.
That's not reasonable for an upper limit. It may be ok for small sites, but not for large sites. I guess you could always run multiple cells, but that seems like extreme overhead just to get more tapedrives.