B+Left combo timing
Note: BN3 runs at 60 frames per second.
The button combo can be executed by pressing B and Left on the same frame. Otherwise, B must be pressed first, then Left must be pressed within 8 frames of pressing B.
If B is released before pressing Left, the combo can still register as long as Left is pressed on the immediate next frame. Otherwise, the window will be cut off on the proceeding frame and the Buster will be fired.
Taking these properties into consideration and counting a same-frame B+Left press as frame 1, this means the maximum length of the window to press Left is 9 frames, and the minimum length is 2 frames. The length is shortened when releasing B, so the minimum length can be achieved by only pressing B for a single frame.