Depending of the visual novel, an After Story can have a different duration from others. I cannot say for sure how long Aokana EXTRA2 is.

Edit: For more reference. I've finished one route in Aokana with Asuka's route in 35 hours, including the common route, letting Autoplay and listening to the characters' voices fully. In EXTRA1, an After Story, I've done the same thing (with autoplay and listening to all voices) and it lasted me 8-10 hours more or less. Given how long the staff worked since they announced I'm willing to bet that the duration of EXTRA2 is longer even for a little.

Edit 2: According to VNDB, Aokana's duration is 43 hours (assuming all routes), EXTRA1's is 6 hours, and EXTRA2's is 10 hours.