Depends. If the story is character driven, introductions are going to follow a character set pacing rather than one that is to serve a plot driven story
It might help to break down the story into arcs to determine when a character should be introduced. If a new character is critical to a relationship with an already established character, having them enter when an arc focuses in on that established character can make it cleaner
The same approach can be useful for plotting arcs. If a character is a main antagonist, introduce them in keeping with setting conflict for the main arc. If a character is a secondary antagonist, introduction can wait until the main arc starts to thread out into smaller pieces and then be woven back in as needed. The character introduction serves to direct the plot
And reader attachment is not necessarily correlated to time spent on screen. Some characters are people you travel with for the whole story. Others are fireworks, there in a flash and gone with a bang but leave the reader wishing for more