Length of hooklength is all about finding the balance between getting them to take a bait and seeing a bite.
It figures that the shorter the hooklength you have, the sooner you will see a bite. This obviously depends on which way a fish moves off with it.
If you assume your 18in hooklength is laid out in a straightish line going away from you ( achieved by a slow pull back immediately after the feeder touches bottom) two things can happen. if the fish moves away from you in any direction you will see a bite straight away, but if it moves towards you it may have to travel 3ft before you see a (drop back) bite. the loger the hooklength then the worse this becomes, a 3ft HL could have to be moved 6ft to register on the tip
The simple answer then would be to use a much shorter HL, but sometimes they will not take a bait that is sat too close to the feeder, and personaly i dont like short HLs as they are more tangle prone and your bait, if its a live one, can end up inside/underneath the feeder
One rig ive been messing about with is to have a short 3 or 4in HL tied onto the line 2 or 3 ft above the feeder, this gives you the best of both worlds, not too close to the feeder, but the bite will register almost straight away. Im not totaly happy fishing this way though. I dont like the feeder trailing behind a fish when netting it, it can get wrapped round the fish or snag the bottom. Also i have trouble with it twisting round the main line so am not confdent my bait is lying free in the water. Suppose you could use a helicopter type set up, but this is not for me. i dont want to be messing around with beads and rings and stuff
this sort of talk probably needs a dedicated thread, im no rigs expert but loads on here are