Good old Nate of Metroid 2002 recently posted his individual level runs of Sonic 2 on Genesis at
SDA, which gets a time of 21:04 by the in-game timer, and about 26:32 total going by the length of each movie file. The currently published TAS of Sonic 2 here clocks in at 21:40, but goes from power on to last input, and the AVI is 25:39.
Level times (Nate's time)
Emerald Hill 1 - 0:18 (0:20)
Emerald Hill 2 - 0:36 (0:39)
Chemical Plant 1 - 0:17 (0:26)
Chemical Plant 2 - 0:51 (1:09)
Aquatic Ruin 1 - 0:19 (0:29)
Aquatic Ruin 2 - 0:38 (0:51)
Casino Night 1 - 0:25 (0:36)
Casino Night 2 - 0:55 (1:10)
Hill Top 1 - 0:39 (0:53)
Hill Top 2 - 0:48 (1:11)
Mystic Cave 1 - 0:28 (0:42)
Mystic Cave 2 - 0:37 (0:50)
Oil Ocean 1 - 0:26 (0:41)
Oil Ocean 2 - 0:48 (1:00)
Metropolis 1 - 0:54 (1:12)
Metropolis 2 - 0:50 (1:17)
Metropolis 3 - 1:30 (2:20)
Sky Chase - fixed length auto-scroller
Wing Fortress - 1:42 (2:05)
Death Egg - 0:42 (1:07)
Those times ain't too shabby. *edited for proper link address*