It's been flat for the past five years but overall, it has lengthened substantially over time. Seems like the big culprit is the number of pitchers used per game per team. So much more specialization among pitchers. Can't find number of pitches per plate appearance over this period of time. It has risen slightly over the past 20 years from 3.74 in 1999 to 3.90 in 2018.
1970 - 2:32 length, 2.66 pitchers per game per team
1980 - 2:33, 2.56
1990 - 2:51, 3.02
2000 - 3:01, 3.54
2010 - 2:54, 3.87
2013 - 3:04, 3.95
2018 - 3:04, 4.36