>>86762192As a veteran rōnin of 4 years, I can say that you don't really choose your oshi. I've seen many people simply realize that they have fallen for their girl without making any conscious decision. Furthermore, when I've seen people knowingly choose an oshi, I've many times seen then committing oshihen after really falling for another girl. That simply indicates that the girl they "chose" was not their kamioshi in the first place.
So, basically, if you really, really like the girl you watch, you can consider her to be your oshi regardless of the amount of time you have spent watching her. If you feel like missing out, you simply have to start watching debuting chuubas, and even that might be waste of time as nothing assures that you would actually like them. I think it's just okay to start liking a chuuba at any point of their streaming career. It's better than being a miserable rōnin for years.