What volume are your drippers outputting? (For example, 1 gph per plant)
For a "traditional" soil/hyrdo flush many folks recommend flushing the plants with 3-4 times the volume of the container you're growing in. So if you've got a 5 gallon pot, you would flush with 15-20 gallons of flushing solution.
With a drip system, you can do a really slow flush, which is kind of nice. It also needs less solution to get similar results.
I do top-drip drain-to-waste, so I don't flush very often. But, sometimes I fuck something up and need to get a clean slate. In that case, my flushing regimen is:
1/4 strength of my vegetative formula
1 tsp per gallon maxicrop soluble kelp powder
1 tsp per gallon epsom salts
1/2 tsp per gallon citric acid
I give each plant about 2-3x their container volume of flushing solution, but I give it to em really slowly, over the course of 24 hours.