There's no hard and fast rules about it. It all depends on what happens in court. If somebody decides you've breached a copyright... even if you've just done a 'similar' sounding song... they can take you to court. Then it's up to the justice system to decide whether you're guilty or not.
To do that, the publisher will bring in a bunch of experts and musicologists to prove their point. Basically, it'll cost you thousands in lawyers fees whether you win or not. Some people take the risk because they know they can afford it cause even if they lose, they still come out ahead.
If you can't afford to defend yourself, don't take the chance.

For instance, there was a commercial recently that used a song really similar to a famous song from the 70's. It had different notes, different chords etc.. but the sound was the same because the drum beat and reverb sounded identical. When you heard it, it reminded you of the original. The producers of the commercial got sued and basically settled out of court for huge $$. Plus they had to replace the fake song with the original one and pay a licensing fee.
They knew if they went to court that they'd lose big time.