I'm doing RA-4 for a while now and I'm still running on my first batch of chemistry that I've mix back in March. I'm still pretty slow at making the color balance so my volume hasn't been that high so far. Nevertheless, a question raised in my mind as how can I know if the developer or the blix as reach exhaustion ? If it make any difference, I'm using the Tetenal chemistry in a processor.
Weak color, and loss of good solid blacks in the shadows would indicate weak, exhausted developer. Milky looking print could indicate bad blix, also a white/yellowish percipitate in the blix tank means the blix is about gone, and if left will get much worse, leaving a yellowish "gunk" coating everything in the tank.
test blix by developing unexposed ra-4 paper in b&w dektol developer. No colour couplers should be formed. Then rinse, and then pop it in the blix. It should come out white after the specified as fresh time if the blix is working properly.
If you're making prints that you need to have the optimal stability I would simply follow the directions on Kodak's documentation for capacities. That is about 16 8x10s or 8 11x14s per liter of chemistry. It works beyond that though.