This setting was removed from Global Settings with the introduction of V2.69. 

In V2.69 and beyond, there is a standard default dim time of 8 seconds. 

Available ToneWoodAmp Mode - Dim (Available on Firmware V2.40-V2.56)

To save even more battery life, the ToneWoodAmp can automatically dim the LCD screen after a
specified period of inactivity.

To manage the Dim setting:

1. Combo-click Effects and Parameters.

2. Turn the Effects knob until you see Dim.

3. Click the Effects knob.

Seconds appears on the display with a value.

4. Select a value in the range 5–20.

5. Click the Effects knob again to select and confirm your changes.