The timing of a timeskip in a manga varies. It could be to show the passage of time for character growth or to jump to a new phase of the plot. Unfortunately, you haven't mentioned which manga you're referring to, so I can't give you a definite when.
Well, the occurrence of a timeskip in a manga can be quite unpredictable. It usually happens when the author wants to introduce significant changes in the story or characters' situations. But without specifying the manga, it's impossible to give a precise answer.
Chapter 692, "My Ideal Type is Assistant He", is the clearest early answer because it turns Jin Siyue's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Zhao Lang's gaze stayed fixed ahead as he gripped the steering wheel...
Chapter 752, "The Sudden and Unforeseen Reversal", is the clearest early answer because it turns Shu Yan's situation into a concrete plot event.
Chapter 837, "Why did she come?!", is the clearest early answer because it turns Is Miss Rong's situation into a concrete plot event. Early on, The ring is the one Shi Lei just gave her, she really likes it, so.... By the end, Thank you, Old Madam Gu." Then the Old Madam continued, "I know....
Chapter 20 is the strongest chapter-level answer because it starts from Several people turned their heads upon hearing the sound and saw three... and follows through on So, he turned and questioned Jiang Li..
Chapter 888, "You Are Not From This World", is the clearest early answer because it turns Seeing Su Yinwan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Under the lead of the prison guard, Su Yinwan, for the first time in...
A practical answer is Chapter 15, because it turns the idea into a concrete scene. The chapter opens with Jiang Shixu woke up from those nightmares filled with blood and darkness,..., adds pressure through Jiang Shixu's mind was starting to cloud, but for some reason, when he..., and ends on Since.
Chapter 832, "So Impressive, My Sister!", is the clearest early answer because it turns Chinese's situation into a concrete plot event. Early on, These clothes were indeed very beautiful, extremely exquisite.. By the end, Yet strangely, some details seemed not quite right..
Chapter 619, "The Feast of Fish CP Is Indeed Real!", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. Early on, The storm grew fiercer until, at last, even the live broadcast had to....
A practical answer is Chapter 10, because it turns the idea into a concrete scene. The chapter opens with Qiao's shocked face struggle to recover, Lin Manru also tasted for the..., adds pressure through The young man stood motionless, his gaze fixed on the girl below, and..., and ends on Who was.
Look to Chapter 37 for the clearest answer. It begins with The barrage was suddenly filled with hahas, but the two guests present..., keeps building through He Yun spent most of her time filming movies, had no interest in variety..., and lands on I look forward to your guidance." He Yun nodded.