I'd say never. So far there hasn't been any evidance or accusation that would really come close. Maybe he did do something that could get him sent to prison, but we need to remember that not the same rules apply to him as they do to us. Far smaller offences would lend us, regular people in prison, but as for him, his so powerfull, hed probably get away with murder.
So far, so good. I believe that Trump has been the most divisive president the US has ever had, but he was also feared for keeping his word. He doesn't fear anybody, and based on his economic success, I don't believe he deserves to go to jail. He doesn't deserve all that, even if I myself don't like him much because of his excessive talking and arrogant attitude toward others who disagree with his opinions. A few days ago, when the FBI entered his premises, I knew it was political propaganda they were doing against that man.