Generally speaking, if you're trying to do everything in Hollow Knight: Silksong, you're looking at 60-70 hours of play. That's a lot of time for an indie Metroidvania, but should you be someone who's already explored every corner of Pharloom, a massive new mod is about provide plenty more adventuring.
The fanmade expansion is known as Hollow Knight: Silksoul, and it provides new areas, mechanics, secrets, and more for the experienced player to unravel and slide through. Right off the bat, what's most intriguing here is the fact that the team have constructed areas that were cut from the released game.