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 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.