[NedL]

Not sure where you get those figures from. A litre of Hypam or Ilford Rapid Fixer at 1+4 is fine for 24 rolls of 35mm or 120 film. Films and RC papers can tolerate far higher levels of silver than fibre based papers without affecting archival permanence.
Ian

Thanks! I was reading this thread and starting to get worried. I've got 500ml of Ilford Rapid Fixer and was planning to develop 12 rolls of 120 and then just assume it's time to replace it. I think I got the 12 from the datasheet, but don't recall for sure. For 135 it's a 250ml bottle 1+4 and I've been replacing after 6 rolls. I do the same with paper... I just keep track of the number of prints and replace when I reach what it says it can do on the bottle.