For many years, scouts/GM's/HC's...have often referred to a 2-3 year rule, in that it takes an average of 2-3 yrs. for many rookies to adapt and develop from playing a position in NCAA to the NFL. This is more true of positions such as WR's, CB's, and some line positions. Many fans however, often expect success from a rookie his first year. Those expectations aren't often realistic based on NFL standards. Now, some say the 2-3 yr. standard for adapting/development under an NFL program...is an old school mentality, since colleges are better preparing their football players for the NFL. So it is debatable, however, this is a very good article defining the different NFL positions and how soon one should expect a rookie to be ready to start successfully. There are exceptions to every rule or standard, of course.
"Official Chiefs Crowd / Historian/Correspondent / Ambassador"
"The greatest accomplishment is not in never falling, but in rising again after you fall. The real glory is being knocked to your knees and then coming back. That's real glory. That's the essence of it." ~Vince Lombardi~