There are a number of different ways it can play out depending on sk many factors but it all depends on who makes the first big move which will determine how much long range and exact rockets hizbullah still has.
It also depends on if the Israeli government and IDF have the balls to destroy most of Lebanon.
War gaming extreme situation
The north is saturated with rocket and missile fire. Attempts are made to take out leviathan gas rig, the haifa Petrochemical area, and multiple power stations farther south. Ramat David airbase is neutralized for some amount of time.
The death and casualty toll really depends on how many people listen to instructions and follow them and stay in a bomb shelter for the time needed. Might be a week or two. .
Depending on who is in charge and level of damage received, the Israeli response is supposed to include the destruction of much of Lebanon, And I imagine ending up with a recreating of the northern security zone pre 2000.