Looks to me like 4 13/16 inches, or bit over 12 cm.
If the center "notch" of the bracket fits inside the tunnel in the tank, you may be able to adjust the holes by squeezing the bracket. That's what I've often had to do. If the notch is too wide, though, then you're out of luck.
You'll also want to check that the holes line up front-to-back. I've had issues getting my Indian copy tank to sit far enough forward, even with the bracket properly oriented. Took a bit of trying but finally got it to mount.