I have successfully used C41 developer that was mixed and stored for just over 6 months. The original colour when mixed was a very pale yellow. After the intervening time it had darkened to a deeper level. I chanced it, an developed a film and it was perfectly normal. Another month and the contents another small glass bottle had gone dark brown so I dumped it.5 years old whether prepared or not I would suggest that the chemical was toast! So managing to use almost the full contents of the kit I think that makes good sense and economy. For this to work, the storage must be in glass and the screw tops must be neoprene plastic
How I managed to make the chemicals still useable after mixing 6 months before was to mix a 2.5ltr kit in one go and decant the working solution into 4 x 500cc glass bottles The remaining 500cc I split into 3 parts and filled smaller glass bottles. They were nominally 150cc, but the 500cc filled each bottle so that there was only a small air gap at the top. Then all the bottles were heated in a microwave to a temperature around 45-50c and the screw tops firmly applied. When the bottles cooled the contents shrunk and formed a partial vacuum inside.
Less air = less oxygen
I know the vacuum was intact because when the tops were opened there was a small inrush of air.
As for the life of bleach and fix, I am still using C41 bleach/fix made from Fuji chemicals and they work as well as new. They must be close on 4 yrs old and not even any sediment in the concentrates.