I've had 3 in my life. One in middle school (and a little after) one in high school, and one just after high school.
Shortest was 2 years, longest was almost 4. Haven't had a oneitis in over a decade now though.
Grotesque said:
The longest was a 4/10 girl that played tennis with me and was nice to me a few times.
Yeah, two out of the three oneitises I had were, objectively speaking, 4/10. I think this is a common thing for men in general, but especially incels. It's partially the illusion of attainability.
The last oneitis I had was a chubby girl with acne. She was in the same social circle as me, and unlike almost every other girl i've shared a social circle with, she was actually nice to me and willing to talk to me. Pretty much every other girl i've shared a social circle with completely ignored me at best, or actively tried to push me out of the group at worst. So naturally having a girl in the same social circle as me actually be nice to me and talk to me led me to falling in love with her.
She got a lot (and I mean A LOT) of attention from guys so I wasn't even going to bother asking her out until a friend convinced me to. Needless to say, she rejected me. Shortly after that she got with a 8/10 guy with rich parents and last I heard they got married.