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# [Raku], 19 bytes

<!-- language-all: lang-perl6 -->

    {(|@_,*-*...0>*)-1}

[Try it online!][TIO-khldvh9f]

[Raku]: https://github.com/nxadm/rakudo-pkg
[TIO-khldvh9f]: https://tio.run/##TcpNCsIwEIbhvaeYlbSlGSY/te2ixXuEIIINCK2VZCFFPZk7LxbTiOAHs5iH9zq4cRemBbYWunDPHvtDWbACEakvcsafwR8XsBlqMiVqbnLoekAtDNjZwXi@DP79wtvsTj5AHBcEUHNIqzaJKL5KfEkmqihWqm0TqUSyqWMK/5WMx7hoEvGVmPj5OvoA "Perl 6 – Try It Online"

~~~
{                 }  # Anonymous code block
         ...         # Create a sequence
  |@_                # Starting with the input
     ,*-*            # With each element being the difference between the previous two
            0>*      # Until it is negative
 (             )-1   # And return the length of the list minus one
~~~