I use Ilford Rapid Fix at the 1+9 dilution for film occasionally, when I have just a few sheets to do. I've tested this, and it works just fine, but the fixing times need to be extended and the capacity of the fixer is reduced. The clip test is your friend here. Test in fresh fix and note the time. Clip-test before every batch. Toss the fix when the clearing time is getting close to 2x the time in fresh fixer. I try to use it only to about 1.5x as a safety margin.
As for fixing time, I fix film for a minimum of 3x the clearing time tested just before the batch. Film is on a waterproof base, so extra time, within reason, will not hurt. I usually fix 320Tri-X for 6 minutes to get rid of the pink coupling dyes. TMY even longer. Both get many more than 3x the clearing time. I use the clip test primarily to test for exhaustion.
Don't misunderstand me here though; I'm not advocating using the 1+9 as a standard procedure; just when you have a very low volume that needs to be done right away. The best and most economical way of fixing film is to collect enough film to use a two-bath fixing regime and the 1+4 dilution. But, you need a bit of film to do this economically. It takes about twice through the change-the-second-to-the-first-bath cycle to get the most out of the fixer.
Note, I never save fixer for extended periods of time like some do; I try to match the amount and dilution of my fixer to the amount of film I have for a specific batch. If I were shooting roll film and developing a couple of rolls every few days, I would likely keep a liter of fix 1+4 going till it reached exhaustion (clip test!) and would never use 1+9. For tray processing sheets, especially using two-bath fixing, saving fixer just isn't practical for me.
I've got several fixing regimes based on the number of sheets I have to develop so I can toss the fix after the session. For just a couple of sheets, 500ml of fixer at 1+9 is fine with extended fixing times. I'm sure I don't use it to capacity; not even close, but I don't toss as much active fixer as if I'd used the 1+4 dilution. For an intermediate number of films, I'll sometimes use the 1+9 dilution in a two-bath regime. Again, I'm wasting fix, but less than if I'd used the stronger dilution. Ideally, I try to collect enough sheets to make using two-bath fix at 1+4 economical. 36 sheets is a great number for me; it uses the fix well and is three batches in my film washer.
Best,
Doremus