How long should a successful rebuild take?

It really depends on how good the coach is. Believe it or not, the Vikings are in a "rebuild year" and look at what they're doing. Its a testament to Kevin O'Connell. Sean Payton's rebuild years had us in the NFC Championship his first year and superbowl win 3 years later with some in between "growth". The Chiefs "rebuild" years were making the playoffs as soon as Andy Reid first got there.

Then look at the Jets and Browns, who have been in a rebuild year since the 90's. Raiders have been rebuilding since Gruden first left, and nearly had it rebuilt when he left a second time, and immediately went back to rebuilding the last many years.

In a nutshell, what I'm saying, is with good coaching and front office there really is no such thing as a rebuild year. You're always competitive. If a coach is struggling year 1 to win half their games, odds are that they will continue to struggle to break .500 their entire career as HC.

This is why I'm down on Loomis. GM for nearly 20 years, and he had the idea that he could be a competitive team with Dennis Allen, and couldn't even recognize that he wouldn't be after two years with him. That's a big sign of a bad GM. Head Coach is more important then any player on the field. And if you look at the Bengals who are stacked with talent offensively, you can argue that Head Coach is more important then any group of great players.