I think it's 4 bars before you have to pay royalties... something like that.
Course that also depends on the ability of the copyright holder to prove his claim in court. ie, if you copied a Beatles riff, you're way more likely to get sued than if you rip of Joe Blows lead solo. There's also other stuff to consider like even if you won your claim in court it won't get you anything if no money was made or lost or no monetary damages were incurred.