If you have maximum black for the film rebate then the chances are you have lost shadow detail by pushing it up into the shoulder of the paper's HD curve.
Of course, if your negatives are very dense from over exposure or over development or 'just because' then you may need to give so much exposure that the rebate does indeed print at maximum black.
Note that this 'max black' advice only applies to contact proof prints.
If you are making final prints as contacts then the exposure should be whatever the image demands, and how the rebate prints is irrelevant.
Print developer lasts until it starts to turn orange or the developing time increases to 150% of nominal.