there is actually a pretty good buffer after the 72 hours is up. missing by a couple/few hours is no big deal.
the 72 hours part is the minimum for when the tomites 'hatch'. but there is still the time period for the protomont to run around at the beginning of that time period (minimum 2 extra hours for you), and for the tomont to harden (minimum 8 extra hours for you). i.e. 82 hours is the realistic maximum between water changes required, but you don't want to edge that, hence why 72 hours is the recommended pushed time period.
i tend to use 76 hours for my personal over/under, but it is all about risk tolerance.