Less than a half week.
I have 5 gallon buckets, so each is its own res.
The bigger you go, the less fluctuation you'll experience...
I'm running canna nutes, and I'm pretty sure that they really don't employ any sort of ph buffers...
so it goes up as the plant feeds past the upper "optimal" pH bound of 6.2, again, in less than a half week. But I do have tiny res's.
The other problem with DWC is as the water level lowers, the roots left out begin to die. Optimally, the whole root system should be under the water, again, a bigger res would help... Which I'll be doing next,
and adding a recirculating pump that has sprayers in the plant bucket, for a DWC Aero set up... and then the water can drain pretty damn low and not begin to kill roots.
Peace.