ShelBl, it happened really quickly. I decided to go cold-turkey on all junk food and sugar for a month and it was pretty hard to do, but I approached it like a personal challenge. At the end of the month, the cravings had all pretty much lost their power over me, which is incredible because I used to be the biggest junk food addict ever, and I never believed I'd be able to kick those cravings.
I was able to go cold-turkey on junk food by doing two important things:
1. I planned every breakfast, lunch, dinner and snack for an entire month so I always knew exactly what I'd be eating, and when. I always kept myself full and satiated, and I never let myself get hungry enough that I needed to "just grab something to eat." It was harder to feel tempted to grab a burger in McDonald's when I knew I had a fully-planned dinner waiting to be cooked at home. By filling up on healthy foods, I didn't have any room left over to eat any junk food. I didn't really love any of the vegetables I was eating, but forcing myself to eat tons of vegetables caused me to not have any room left over for Twinkies and hot dogs.
2. I learned a lot about the food industry, and all the horrible things that food companies do to our food, and what those food additives were doing to my body. I lost my ability to block out the fact that junk food is, basically, toxic rotten nastiness. I couldn't get past these thoughts about all the horrific things McDonald's does to their beef (google "pink slime"), and I couldn't get past these thoughts about the havoc that trans-fats wreak on the human body, and I just couldn't get past these thoughts. I couldn't look at a piece of junk food and think of it as a "delicious, yummy treat" anymore. By educating myself I just came to see junk food as disgusting, nasty, icky, vomit-inducing crap. I literally want to throw up when I think about the stuff that gets marketed to American consumers as "food." Most junk food barely even qualifies as being "edible." This has been a huge, huge, huge source of motivation to steer clear of junk food. I don't even want to put that poison anywhere near my mouth anymore.
So, it took around 3 weeks of going cold-turkey for my physiology to change. My entire system started changing and working differently. I went to an amusement park a month after starting, and I treated myself to a big bucket of cheese-fries, and the cheese-fries didn't stay down for very long. Crazy. My body can't even deal with straight-up junk food after a month of going cold-turkey on it.