It’s always best to keep an ongoing conversation.
Remember, there is no “right” time, your boss will always be busy, so it’s always better to bring it up sooner rather than later.
Also, you don’t have to have a pitch/goal the first time you talk about it.
Just tell them you want to grow and want to know what they think and what they recommend to help you do that.
Note: if you’re asking for a position change/raise, it’ll be a different approach.
Experience: Engineer that went from intern to engr 1 to engr 2