I'm no expert on any of this. Both Ilford and Kodak give similar exhaustion rates for their fixers and these are undoubtedly conservative figures due to many unknown factors once the product is in the hands of the consumer. Other brands probably have similar exhaustion rates but I would consider dumping them a little more often than either Ilford or Kodak unless there are manufacturer's recommendations available that state otherwise.
Personally, I use film strength, non-hardening rapid fixer as a two-bath solution for prints. With the initial chemicals, I dump the first bath a little more often than is recommended and move the second bath up to number one. This rotation keeps the second bath fresh and the first bath used to near exhaustion. With film fixer, I test the chemical with a film leader or dump the fixer after six months or so. The amount of film I shoot and process can vary considerably over time. Sometimes I shoot dozens of rolls in a month and sometimes I shoot nothing for several months.
As for the one-shot fixer idea, I make no sense of the notion for dumping out perfectly good chemicals, especially when based on an environmental/conservationist argument. It's wasteful and it simply means the consumption of more chemicals and more chemicals being dumped into the environment in the long run.