EDIT: Magnussen spent 6 years in the junior ranks before being promoted into F1. That's a pretty standard length; Hamilton also spent 6 years in them, as has Vandoorne. He was probably put into F1 because he looked ready, and McLaren needed/wanted to replace Perez.
Palmer and Nasr are perfect examples of this
Palmer is a perfect example of the opposite. Talented drivers adapt and are quickly able to show their talent. Palmer did not, he was a slow starter in every series he entered and his GP2 career was terrible.
In 2011 he entered the 4-race GP2 Asia series, finishing 19th overall scoring zero points. Later in the year he entered the main GP2 series and he finished 28th out of 32 overall.
28th place, again scoring zero points and was the lowest placed driver to participate in every event.
In his second GP2 season - or third if you count the GP2 Asia series as a 'season' - he was better, but still only finished 11th, and was 7th in year 3 (or year 4).
His Formula 2 career was a little bit better, in his second season he finished 2nd overall, but finishing 21st overall in his first season was still dreadful. In Formula Palmer Audi he was 11th on debut and 3rd the next year.
Nasr had a strong junior career, even if his GP2 stint was only decent (10th, 4th, 3rd) the rest of his career was pretty good.