Essentially you want to remove this after it had been added, but before that hook fires. Which is hard to tell in general without knowing when does the parent theme executes this.
Also in often confusing way functions.php of child theme is loaded before that of parent theme. Often makes timing of things counter–intuitive.
The very general practice is that you wait until init hook to run any logic, other than strictly related to the load process. So my guess would be on/around init, but really you should figure out when is it added and decide based on that.