How close can a trademark be to a famous brand?

It's hard to definitively state what is and what is not considered to be an infringement of a mark - on its face, it would seem that "forever 18" and "forever 21" would be confusingly similar if in fact they were marks used in the same field of goods or services. The rule of thumb usually is that the more famous the mark, the more well-known the mark, the more protection it would receive. For example, while everyone knows Google, and while Google originated with search services, Google could effectively today enter into any field and people would immediately associate the particular goods or services with the original Google corporation. In a similar manner, if someone else started using the Google brand/mark for even diverse goods or services, the real Google would probably prevail in an infringement suit because the mark /brand Google is so strong, therefore it is accorded extremely wide protection.