Day tank dilemma

Over at fab 86 an incident occurred where a batch of HCl at pH 2 was accidentally prepared. Unfortunately, however, it was desired to have material of pH 3. To accomplish this conversion, the shift decided to flow in acid of pH 4 all the while emptying fluid from the well-mixed day tank at the same rate. How long will this take?

Contributed by Paul Safier, Intel Corporation.