[halostatue]

Not all jurisdictions are the US, and not all jurisdictions allow fair use, but instead have specific fair dealing laws. Not all jurisdictions have fair dealing laws, meaning that every use has to be cleared.

There are simple algorithms that everyone will implement the same way down to the variable names, but aside from those fairly rare exceptions, there's no "maximum number of lines" metric to describe how much code is "fair use" regardless of the licence of the code "fair use"d in your scenario.

Depending on the context, even in the US that 5-second clip would not pass fair use doctrine muster. If I made a new film cut entirely from five second clips of different movies and tried a fair use doctrine defence, I would likely never see the outside of a courtroom for the rest of my life. If I tried to do so with licensing, I would probably pay more than it cost to make all those movies.

Look up the decisions over the last two decades over sampling (there are albums from the late 80s and 90s — when sampling was relatively new — which will never see another pressing or release because of these decisions). The musicians and producers who chose the samples thought they would be covered by fair use.