A player qualifies to lead the league in "rate statistics" (batting average, earned run average, on base percentage, et al.), when he averages 3.1 plate appearances per team game for hitters or one inning per game for pitchers.
If a batter does not qualify, he may still win a title if his rate would lead the league if his record were padded with enough hitless at bats for him to qualify. No equivalent standard is applied for pitchers' records.
In a 162 game schedule, a player needs 162 innings or 502 plate apparances to qualify.
In 1996, Ellis Burks had 211 hits in 613 at bats and 685 plate appearances, thus qualifying for the title with at .344 batting average.
Tony Gwynn had 159 hits in 451 at bats and 498 plate appearances. That was good for a .353 batting average and a .349 batting average when his record was padded with the 4 hitless at bats needed for him to qualify for the title.
Gwynn's batting average was higher than Burks's even after being charged with enough hitless at bats to qualify, he was declared the National League batting champion. The record books recognize him as batting average champion with his actual average of .353, not the .349 average that he would have had with the extra hitless at bats.
This standard has changed over time and the current standard was put in place in the 1950s.
There is no strict standard for the league leader in winning percentage; some sources use 12 decisions, others 15, and others yet a different number.