Does anyne here mix their own fixer powder?

[slashmaster]

I'm looking for a little help to get the most out of or even make my own fixer. I'm first of all still confused about how long you can use it. I already know to use a test strip to see how long it takes to clear and I know with each use it can take a little longer each time. But how long is too long? I get the feeling letting your film sit in fixer 2 hours so you can get more moneys worth from your fixer is too long? Also what are the ingredients if you want to mix your own fixer powder? Is it only sodium thiosulfate and sodium sulfite? Would just those 2 ingredients and water work? I'm starting to get sick of paying $15.00 or so for a Kodak bag of fixer but I see a 10 pound bag of sodium thiosulfate for $25.00