That depends on your VCO; or whatever you are driving from VTUNE.
Typically, a VCO will have an input voltage range and a corresponding output frequency range. For example, the input voltage range could be 0 - 5 V. The output frequency range could be 1000 - 1050 MHz. This means VTUNE = 2.5 V would give an output of 1025 MHz. The gain of the VCO would be 10 MHz/V.
Do you know the gain of your VCO (or laser if I remember correctly)?