Hollow Knight: Silksong's Steam achievements are live and reveal the game seems to be about 50% longer than the original.
Hollow Knight: Silksong just released, with no pre-release codes going out for press or Kickstarter backers; we're all experiencing it at the exact same time – and because of that we have no clue how long the game actually is. However, comparing the game's Steam achievements may help us estimate just how long Hollow Knight Silksong is.
In the original Hollow Knight, there are three speedrunning-based achievements: one for completing the game in under 10 hours, another for under 5 hours, and the big one – completing 100% in under 20 hours. Silksong continues the trend with speedrun achievements of its own, for beating the game in under 5 hours.
There's also an achievement for "Speed Completion," which is interesting. You'll need to "Achieve 100% game completion and finish the game in under 30 hours," as opposed to the 20 hours required by the original Hollow Knight.
This could be me thinking too much into it, but based on the difference in time for the achievements, it's probably safe to assume Silksong could be roughly 50% longer than the original.
How Long To Beat lists Hollow Knight as taking 27 hours to mainline (I beat it in 18 with some extra exploring, so that doesn't seem right...), with 100% completion taking about 64 hours. So if it's as exact as the achievements suggest, Silksong could take about 96 hours to beat 100% on an average playthrough, but I'd wager it'll end up being a bit shorter than that.
