GHB is in powder form naturally - the dose depends on how much GHB has been dissolved per ml of water. Or do you mean GBL? Is the liquid you have slightly salty, or does it have a strong stinging solvent taste?
Start low, give it plenty of time between inter-dosing as GHB can take long periods to kick in, as two overlapping doses could spell an overdose for a GHB naive individual.
^This
As the others have said its a bit of a lucky dip with GHB. If you trust whoever you are getting it off, I would start by asking them about the effects of different dosages of those who have tried that batch as well as their tolerance so that you have a rough idea of the range you're playing with.