When Martin Luther was just short of sixty years old he pleaded with pastors to be diligent and not lazy:
"Some pastors and preachers are lazy and no good. They do not pray; they do not read; they do not search the Scripture ... The call is: watch, study attend to reading. In truth you cannot read too much in Scripture; and what you read you cannot read too carefully, and what you read carefully you cannot understand too well, and what you understand well you cannot teach too well, and what you teach well you cannot live too well ... The devil ... the world ... and our flesh are raging and raving against us. Therefore, dear sirs and brothers, pastors and preachers, pray, read, study, be diligent ... This evil. shameful time is not the season for being lazy, for sleeping and snoring." 
"Let ministers daily pursue their studies with diligence and constantly busy themselves with them ... Let them steadily keep on reading, teaching, studying, pondering, and meditating. Nor let them cease until they have discovered and are sure that they have taught the devil to death and have become more learned than God himself and all His saints." 
which, of course means never.
Here is John Piper, pleading pastors to not waste their pulpits, but rather use it for radical bible saturated preaching, to see people ignited with the Bible.
 Meuser, Fred W., Luther the Preacher, pp. 40-41
 What Luther Says: An Anthology, Vol. 2, p. 927.