No matter how long the slinky is, the bottom of the slinky will stay still (hover) until the top reaches it. Even if the slinky is over 1000 feet long.
This was a fantastic episode of Veritasium. Maybe the best. The recoil of a slinky, stretched under its own weight, is a pretty fantastic physics problem, especially when you capture it in super-slo-mo.
Check out the original video here. And the analysis here. And of course Derek had to take it to another level and drop a slinky from the top of a building.