I would just about totally agree with the advice given by Doremus, the only difference that I have in my approach is that I do not time prints in the toner. Prints being toned in selenium toner respond differently depending on tonality, therefore I believe that the time for a high key print will differ from a very low key print. Low values in the print will begin to change colour before delicate higher values, that is one way to note that the selenium is affecting the image, the blacks will also get slightly darker as the toner starts to work.

Toning is very subjective and needs practice to learn how to control it. If you do happen to give a print too much selenium toning and end up with an unpleasant almost purple print you can recover this by washing and toning in gold. This will cool the mid tones and highlights and bring the lower values back to a more acceptable "plummy" colour. Gold affects highlights and mid tones before lower values which is a useful tool when split toning images.