I've looked at the AP final regular season rankings since 1936 - a little over 80 years. Using the following logic - a top 4 finish in the regular season, 5th if a second school from a conference is also ranked, the ACC and it's predecessor Southern Conference would have placed the following teams in a 4 team playoff:

38 - Duke
41 - Duke
43 - Duke
48 - UNC
51 - MD
53 - MD
55 - MD
Twenty year hiatus due to the 800 SAT Minimum Rule
76 - MD
81 - CU
90 - GT
91 - FSU
93 - FSU
94 - FSU
95 - FSU
98 - FSU
99 - FSU
00 - FSU
01 - MD
07 - VT
13 - FSU
14- FSU
15 - CU
16 - CU
17 - CU
18 - CU
19 - CU

That would be 26 appearances over 82 years - roughly once every four years.

The main source of criticism for the ACC is ignorance of the past. The Pacific Conference brought along it's pre-1959 records whereas the ACC left their pre 1954 records behind. That obscures how good Duke, MD, Clemson, and UNC football were in that time frame. Most folks outside the ACC don't understand what happened in 1962 when the 800 SAT Minimum Rule was adopted and how it negatively affected ACC schools for 15 years causing recruits that would have played in the ACC to no longer be able to get into the ACC and thereby end up at Penn State, WVa, Georgia, Auburn, TN, etc., . Keep in mind that in the 1960's a black student in the South had a difficult time making 800 on the SAT due to generations of an inferior education.