Sean O'Rourke has the FKT at 16h05m, August 3, 2014.
Challenger is at the northern end of the Pickets, a remote subrange of the north Cascades. The climb requires a fair bit of glacier travel and a few moves of 5.5 rock near the summit. I dayhiked it from Hannegan Pass in 16h05m, going in via Easy Ridge and out over Whatcom to Whatcom Pass. I believe this time could be substantially improved, but am unaware of other efforts to dayhike this peak.