It was gradual for me. Probably all started with yuengling, then boston lager and blue moon.
My first DFH 60 min probably sealed the deal though. I took it from my girl friends room mate. I had heard about it and wanted to try it, and some kid brought over a 6er of it (probably stole it from his parents) had one and left the rest in the fridge.
My gf though had a very abrupt change when she had her first St. Pauli Girl. Before that she wouldn't drink anything but coors light, but now she likes pretty much all of my beers. Its funny because I don't even really like st. pauli