10 seconds of the destruction process.
The jumping time of Shanks, & the time before jumping (just thinking of FS) arent being counted i guess. So he might be can do > 10 s
This tbh.
When Shanks jump, the cannon hasn't been fired. Let's say this is the preparation time.
Sanji can move those distances in 1 second (egghead saving Edison), Shanks can do the same. Especially that Marineford Akainu scene. So let's assume Shanks jumping & attacking = Cannon preparation time = minimum 1 second.
The entire destruction is 10 seconds. Shanks' FS witnesses until the end, where the 2 giants died.
So, the FS saw the entire cannon prep time (the time Shanks moves & attacks) + 10 seconds (of destruction). At least 11 seconds.
If anime Roger's Divine Departure is used as basis, more than 11 seconds.