I am assuming you are asking about extract, usually at the beginning but you can add half of your extract in the last 15 min to prevent burning the malt and getting darker colors and an unwanted roasted flavor.
Go to Brewers friend and put in your Malts and hops. There's an option for late additions to see what your IBUs will look like. It can change them a lot.
I find that if I have half "dry" and half "liquid" extracts i like to boil the dry first and the liquid later as it scorches a bit easier and tends to darken in general "shelf life" a lot faster than dry extract