For most purposes the recommended dilution for Jeyes fluid is 250ml of the stuff to 5 litres of water, so you could try mixing it in that ratio and then pouring down as much as you need.
However asking how much Jeyes fluid to put down a drain is a bit like asking how much bleach to put in your toilet pan to stop it smelling. The answer is "enough to do the job"! Simply pour it down (probably after diluting it but, if there's plenty of water down the drain you could tip it in neat) until the drain no longer pongs!
Chris