There are two parts to your question: 1. how long to count the ballots, and 2: how long before we know who won the election.
The physical counting of the 7 million paper ballots which are cast in person will take about 1 day. But it will take 2 days before the final totals are announced ( because about 10% of the ballots are absentee, and take longer to verify and count.)
But the physical counting is only one tiny step in the process. The real question is who actually wins the election. (i.e. who can convince 61 out of the120 newly elected members of Parliament to join a coalition.) And that will take at least one month, and probably 2 months before we know the answer.
The process goes like this: The president of Israel is a strictly ceremonial position, with no political power, except for one very vital function. After an election, the president officially decides which party leader he will nominate for the top job of ( attempting to become) prime minister. That person then has one month to negotiate with all the other parties and convince them to support him as prime minister. If he succeeds in convincing 61 people of the 120, then he has won the election.
If he fails to gather 61 votes, then he returns to the President and officially announces that he has failed. Then the President nominates the other major party leader, who is then given one more month to gather 61 supporters. If he succeeds, he has won the election.
Right now, (4 hours before the polls close) it appears that neither of the two major leaders has the support of 61 members. But it looks like the first nomination by the President will go to Eisenekot. He is a nice , soft-spoken guy with very little experience in hard-ball politics, and it is quite possible that he will fail to cajole 61 supporters. So, a month from now, on November 27, the President will offer the job to Bibi Netanyahu. Bibi is a truly masterful politician, a hard-ball, conniving manipulator who may well succeed in winning 61 supporters. He will draw out the coalition negotiations for another full month, making last-minute promises, etc, until the deadline of December 27..
So–the answer to the original question “how long ?” is : either on November 27, or on December 27.
If after that both sides have failed to gather the necessary 61 supporters, new elections will be called , on March 27 (i .e. after a legally required 6 month break).