This is a tough question to ask given that as the author, you can introduce the conflict at any given point that you wish.
I would suggest within the first 10 chapters so that the readers have time to understand what is going on, where the story is going, and be able to properly interpret the scenes. But then again, how/when you introduce the conflict is no concrete subject. You can introduce it instantly with a harsh environment/setting, or with the simple introduction of a tricky character. You have room to play with it.