Without knowing what mods you have, I'd shift it around 5k. You gotta just experiment with how the car feels at high rpm's. Depending on what you've got installed on the car, it could be sooner or later but 5k should be safe. Maybe 5.5k if it feels like it wants to keep pulling.
Try to get to the point where it doesn't drop less than 3k rpm's after you shift to the next gear. That should atleast keep you in the power curve.
I've got 4.10's in my car, so I have to shift a bit sooner than normal.