I have been dumping as much as 100 gallons every other week into my septic tank for 28 years and have seen zero problems, EXCEPT, the area where my drain field runs under my lawn, seems to start going brown sooner in the summer before the rest of it goes brown.
I generally water up through the middle/end of July before getting tired of the water bills.
But it always greens up with the rest of the lawn.
But there have been no problems with the septic tank doing it's job.