Yeah ur ph will drop, that’s cos there’s not enough nutrients left in the solution.
A greater amount of solution in the first place will see you through that bit longer without having to adjust. Il be honest I adjust once a week And add fresh tap water or change res every 2 weeks. When the plants are in veg and they’re getting little and often feedings I’ve seen me go 4 weeks with only 3 adjustments and 1 top up of fresh water, and I know from experience I could use an even bigger res and not have to touch it for a month.
I’m not sure how long a and b nutrients would last mixed together without reactions rendering some compounds unavailable to the plant but I know from experience that what I do works. That’s all I need to know really, my plants never want for anything and they’re allways growing at optimum rate.
Temperature plays a part too, the colder means more oxygen, the warmer means more efficient root function and nutrient uptake. My happy medium sits slightly on the warm side, at a controlled 21’c. I use an aquarium ozone generator and bleach for pathogen control though, bennies would probs do a better job in a cooler solution though. Just some food for thought