When did the disciples first find out that Judas would betray him?

Mk 3:

16 These are the twelve he appointed: Simon (to whom he gave the name Peter), 17James son of Zebedee and his brother John (to them he gave the name Boanerges, which means “sons of thunder”), 18Andrew, Philip, Bartholomew, Matthew, Thomas, James son of Alphaeus, Thaddaeus, Simon the Zealot 19and Judas Iscariot, who betrayed him.

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As readers, we knew early on that Judas Iscariot was the betrayer. We knew right away. They didn't.

In the final year of Jesus' ministry, J 6:

70 Jesus answered them, “Did I not choose you, the twelve? And yet one of you is a devil.”

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Jesus informed the twelve that one of them was a bad guy. He didn't use the word "betrayer". They didn't know the significance of Jesus' statement.

71 He spoke of Judas the son of Simon Iscariot, for he, one of the twelve, was going to betray him.

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This was dramatic irony. There was no indication that they confronted Judas or treated him differently after this. At this point, they still had no idea of a possible betrayal until the Last Supper. Mt 26:

20 When it was evening, he reclined at table with the twelve. 21 And as they were eating, he said, “Truly, I say to you, one of you will betray me.”

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That's the first time the disciples heard the word "betray".

22 And they were very sorrowful and began to say to him one after another, “Is it I, Lord?”

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Each wondered if he would betray Jesus.

Parallel account, J 13:

21 After saying these things, Jesus was troubled in his spirit, and testified, “Truly, truly, I say to you, one of you will betray me.” 22 The disciples looked at one another, uncertain of whom he spoke.

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They wanted to know the identity of the betrayer.

23 One of his disciples, whom Jesus loved, was reclining at table at Jesus’ side, 24 so Simon Peter motioned to him to ask Jesus of whom he was speaking. 25 So that disciple, leaning back against Jesus, said to him, “Lord, who is it?” 26 Jesus answered, “It is he to whom I will give this morsel of bread when I have dipped it.” So when he had dipped the morsel, he gave it to Judas, the son of Simon Iscariot.

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Jesus implied that Judas was the betrayer.

The next question was: how would Judas betray Jesus?

27 Then after he had taken the morsel, Satan entered into him. Jesus said to him, “What you are going to do, do quickly.”

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Now, they knew the identity of the betrayer. Still, they had no idea how Judas would betray Jesus.

28 Now no one at the table knew why he said this to him. 29 Some thought that, because Judas had the moneybag, Jesus was telling him, “Buy what we need for the feast,” or that he should give something to the poor. 30 So, after receiving the morsel of bread, he immediately went out. And it was night.

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As readers, we knew Judas Iscariot would betray Jesus as soon as Jesus had chosen the twelve. But the disciples had no idea of that.

They first learned of the possibility that someone among the twelve was a bad guy, not necessarily a betrayer, in the last year of Jesus' ministry.

During the Last Supper, for the first time, they learned that Judas was going to betray him. Still, they had no idea how Judas would betray him.

After the supper, Judas Iscariot led a group of soldiers and officials to the Garden of Gethsemane to arrest Jesus (J 18:2). Finally, the disciples saw the act of betrayal.

The narrative unfolds with a gradual increase in the disciples' understanding while maintaining dramatic irony for the reader throughout.