You answered your own question in the OP, you know what the motor is rated for yet asked how long before it will fail so I helped put things into perspective :)
FWIW, I once ran a similar motor that was rated for 3S on 4S and I had some amazing speed runs, I think I got maybe 4 speed runs until...
Eff around and find out how many seconds it takes to delaminate the magnets, looking forward to the follow-up post on where to buy a new motor which will likely be the end result regardless since you have the need for speed...