How long into bulimia does your teeth start to be affected?

Immediately: Bulimia affects your teeth the moment the acid from your stomach touches them. You probably won't notice the affects until much after. The acid in your stomach is hcl acid. It is very strong.

Depends on frequency: When you vomit, the contents of your stomach including digestive acids contact the back of your teeth. These acids literally dissolve your tooth structure. So in reality, the process starts each and every time you vomit. Please go see your physician and or psychiatrist for help. There are many ways to help you, but you need to realize that you have a significant problem and want to help yourself!