Ilford say that 1 litre of their Rapid Fixer should be sufficient for 24 rolls of 35mm film. I've read somewhere that a roll of 35mm has about the same surface area as a sheet of 10x8 (I haven't done the maths though), so it should also last for about 24 sheets of 10x8. However to be safe I mix up fresh fixer after 15 sheets of 10x8 (i.e. about 15 rolls) which works out at about 10p per sheet/roll. Maybe I'm using more fixer than I need, but it's not the most expensive stuff in the world and I'd rather be safe than sorry.