Kathryn, there's no rule on when to introduce a Nemesis.
More important than WHEN you introduce a Nemesis is WHAT the nature of their relationship is to the Protagonist. Bonus points if the…
Kathryn, there's no rule on when to introduce a Nemesis. What kind of character they are... what type of genre you're writing... who your Protagonist is and what the nature of their journey is...
More important than WHEN you introduce a Nemesis is WHAT the nature of their relationship is to the Protagonist. Bonus points if the Nemesis represents some aspect of the Protagonist's psyche which they fear confronting.
Rather than be concerned about what a development executive may think, write the story YOU want to write the way YOU feel it needs to be written.
Good luck!