Depends. If I'm just running base not much drift at all. I usually start out at 5.5 and let it go up to 6.0-6.2 before knocking it back down. Usually just once over a week and by the time it's back up it's time for a new batch. I usuall add tea halway through the week at 2 cups per 10 gallons. It drift up faster with the tea.
Now if I use the whole H16 line(roots, final and prime) it drifts more. Mainly because of the Prime i think. Also the lower my res gets the more it drift up, I have a lot of air in the res. Water gets warmr faster when it lower which also raises ph.
I just ran out of Aptus so I'm on the whole H16 line this week. I'm sad to say I dont think the girls are as happy. I've been having to use a higher base ppm w/o Aptus also. Don't get me wrong they're doing great just not quite the same. Hope that helps.
pH may drift more when organic inputs are implemented. Mineral only formulations such as H16 base nutes, should hold a steady pH in a reservoir unless there is a root disease or build up in the rhizosphere.
I also recommend to implement drain-to-waste techniques as much as possible to ensure consistent nutrient irrigation.