Perfect SBR length

[interceptor]

Hypothetically speaking, if a guy was going to cut down an AR barrel and do a SBR, about what length would be desireable? Then end result would be something I would want to use for out to 100 yards, subsonic with the can.

I see some others that are about 10.5" here on the board, but want some imput as to how those guys feel about them, if they would have gone longer or shorter if they had it to do over again.