bath waste question

I am about to install a new bath and was hoping to put the trap under the timber floor ( the house is on stumps). My question is how much of a fall (how long a piece of pipe) can you have from the bath waste to the trap without creating so much momentum that the water flushes straight through the trap? I hope this makes sense!!
Cheers

You could have forty feet if you wanted to....water will always be caught in the trap no matter how long the vertical drop is

25mm - 35 mm per meter is what your looking for. if you have more than that just set the pipe down with 2 -45 degree bends.