In his book Outliers, Malcolm Gladwell argued for what he calls the "10,000 Hour Rule." He said that one of the greatest contributors to success at a skill is 10,000 hours of practice. People develop great expertise in large part due to vast amounts of practice.
Gladwell may have been wrong or he may have overstated the importance of practice. According to an article written by 5 psychologists and published in the scholarly journal Intelligence, practice can only do so much to advance an individual's skill level. Douglas Main writes in Popular Science:
In the study, authors re-analyze scores of studies on elite chess players and musicians, but especially the former, since every player has an easily quantifiable numerical rating. They point out that there is enormous variation in how long it took for people to get to the level of a chess master. One player in a 2007 study, for example, "took 26 years of serious involvement in chess to reach a master level, while another player took less than 2 years to reach this level," they write. They conclude that practice can only explain one-third of the variation in sucess in chess and music, and probably other fields as well.
The evidence is "quite clear that some people do reach an elite level of performance without copious practice, while other people fail to do so despite copious practice," they write. They suggest that other factors together explain the lion's share of success (at least in these two most-studied areas), such as intelligence, starting age, personality, and other genetic factors.
This is especially true if you're playing a multi-class character.
-via Fast Co Design