It happened over time for me, starting at puberty and developing before becoming something fully realised in my university years.
There were two realisations. The first is that you're not that attracted to what all your friends are. You're in high school, and your friends are all talking about which girls they think are hot and, while you certainly can appreciated their beauty, they're clearly not "doing" for you what they are to your friends. The next is the realisation that you like bigger girls.
Two examples here stick in my mind. In 9th grade, we had a substitute teacher for a few weeks in maths class and she would have been in her 20s. She had a very curvaceous body and I thought she looked amazing as opposed to my friends who just thought she was "fat" with negative connotations.
The second example was in 11th grade, at the start of the year. A girl in my year had gained a very noticeable amount of weight over the summer break, most of it seeming to have gone to her ass and thighs. I thought she looked amazing.
As high school went on, my tastes evolved and I went from wondering what was wrong with me, liking curvy girls, to, by the time I reached college, completely at ease with myself and my preferences, which by that stage had evolved to girls much larger than at the start.
11 years