When exactly, none of us here can say. But it is certain that it requires considerable advancement which means for most people several decades, or more, even to the next lifetime(s) because the time might not just be yet.
Your best route of action is to keep consistently growing and consulting your Guardian or other Demons regarding this topic.
The magnum opus can take anywhere from a whole lifetime, to decades, to multiple lifetimes. There is no easy answer to this because every soul is on a certain level of development.
Definitely just focus on your advancement. When you are ready for the magnum opus the gods will let you know.