I don't think I have ever heard Lea refer to herself as a "princess" however that was how people refereed to her in the original trilogy.
I guess that she officially lost the title when Alderan was reduced to a heap of smoldering rubble.
However in The Empire Strikes Back Han is happy to tease her about it. So it can't be that sore a point at that point in time.
In The Force Awakens we have this dialog.
Lor San Tekka: Oh, the General? To me, she is royalty.
Poe Dameron: Yeah, but don't call her Princess, not to her face. She doesn't like it anymore. Really doesn't like it.
(Dialog from the novelization)
Now, I'm assuming that she doesn't like it as that is how Han referred to her.
Do we have any indication of when it was decided that 'Princess" was not her formal title any more?