Quite a few ways to achieve this, but the simplest ways are as follows.
Take the current length dimension of you object and calculate the ratio as:
ratio = 120 / current_length
Then press the S key then type in the ratio into the number pad and hit Enter.
Another way to do this is press the S key then press the / key then type in the current_length value, then press enter, this will scale the object to be 1mm long. Then press the S key and type 120 in to the number pad and hit Enter to scale the object up to 120mm.
The other option is to just hit the S key and move the mouse until you are roughly around 120mm then hold the Shift key to give you fine grained control of the scaling and set to exactly 120mm.