4 years can be really impressive but its so rare, I don't even know if we've seen anybody like it recently.
5 years is considered the norm by those senior in my department. Not by those more junior or in middle ranks.
6 years is considered the norm by many of those more junior in my department (some of them took 6 years). So sometimes there is a bit of debate over whether a 6 year is penalty, and usually the consensus has been no. But its debated occasionally as a tie breaker. When you have to narrow down 30 interviews, and 15 were really good, down 3 flyouts, people are just looking for ways to make choices. So again, some people bring up the 5-6 year stuff, but usually its chalked up to departmental culture. So many departments are set up to get people out in 5. Others practically encourage you to be done in 6.
7 years, 8 years, if the candidate looks great otherwise, we look at the letters to figure out what happened (medical, family stuff, pre doc, etc.).