You can turn the lights on, from the moment the print hits the bleach. It is acidic enough to prevent any more developing. You may then see when enough bleaching has been done. After enough runs, you can tell when the development has been correct too.
There is no chemical reason not to use sodium bisulfate (sodium hydrogen sulfate). It is a safer alternative to concentrated sulfuric acid. Either bisulfate or dilute sulfuric acid are capable of providing a low pH for the bleach.
Originally Posted by spoolman
Sulfuric acid can be purchased in dilute solution 10% to 51% as battery electrolyte. Any store that sells batteries should have it. The percentage of sulfuric acid should be given on the label.
The secret to BW reversal processing is to judiciously follow a method that is known to produce good results. Another tip is that everything thing works backward. If your slides are too light then you must decrease camera exposure.