As others have said, drama shouldn't be something you "insert". It will feel more natural and less like "stalling the plot" if the ingredients for it are part of the story itself. It should be less of a question of "when" (as if you can just sprinkle it in wherever you want...) and more of a question of "why" and "how".
First of all, what kind of conflict do you want? If it's the usual physical conflict, then you should consider the physical obstacles your characters could face-- natural phenomena, monsters/wild animals, enemies who want to hurt them, enemies they want to hurt. Where are these things in the fictional world, how and why could the characters encounter them?
Once you figure that out, you can build a story that will get your characters into those situations, and the speed at which that happens will determine the "when".
Now, if it's interpersonal/emotional conflict, that's when things get trickier. Time and space don't necessarily have to be factored in here; bad feelings can come up whenever or wherever. What you should focus on is cause and effect.
You can start the same way as the above, thinking of the mental obstacles your characters could face-- a prideful character might come up against an embarrassing failure, or a cowardly character might come up against a difficult decision they can't run away from. Who or what could cause these situations to arise, and what effect would that have on the character in question? How would they choose to respond to it, and would their choice make things better or worse? (If you're looking to create drama, I'm assuming you want 'worse'.)
And then, what effect will that initial response have on the rest of the story, and the character's behavior? How will they try to solve the problems caused by these effects, and so on and so forth?
You have more control over the "when" in this case, but that comes with more responsibility as an author, and a greater awareness of how the pace of these conflicts will affect the reader's experience. In general, I think it's a good idea to wind your biggest dramatic moments around the most important parts of the story, the ones you really want your readers to pay attention to.
Like, if the climax of the story is the character's confrontation with their long lost mother, that should be a spot where you consider creating drama. Maybe the character hates their mother for leaving them; maybe the mother wants to keep the character away from them and is too ashamed to tell them why. Maybe a third party is forcing them to hurt each other when neither of them wants to.
To give a different example, if there's a point in the story where the character needs to learn some important information, make it happen in a dramatic way. Maybe they learn it from someone they never thought would want to help them. Or maybe they find out just before they're ready to give up, and it completely changes their mindset, motivating them to continue furthering the plot.
Basically, when all else fails, you can use drama as a sort of 'highlighter', where it simply follows the pacing of the story around it, and makes the crucial events stand out the most.
