According to my Trainerroad power curve for this season (so since I got back off holiday in middle of Jan), I’m at 1 minute 3 seconds for max duration of 5x my weight today.
My all time record at that power is 2 minutes 29 seconds. So yeah, 5w/kg is never going to happen!
Looking with the same mindset but with 4x, my current season best is 3:12 and my all time best is 8:35. Which shows how much of a stretch 4w/kg on an FTP basis really is for me.