During a chat with Famitsu at the Tokyo Game Show recently, producer Shun Nakamura mentioned how the new Shadow-themed adventure would apparently be "about the same length" or possibly "a little less" than the main Sonic Generations offering (translation via ResetEra).
According to How Long To Beat, the original Sonic Generations' main story is roughly six hours in length on most platforms, with "mains + extras" 12 hours and a completionist run taking about 21 hours.
This latest update follows the news SEGA will also be adding a special themed DLC level featuring key scenes from the upcoming Sonic 3 movie. In this chapter, Shadow will be voiced by Keanu Reeves.
