An example of a plant mature enough to take a stem cutting from:
As you can see, the plant has made a new growth (often referred to as a “basal”) at the base, and so by taking a cutting off the original stem, the ground shoot will be encouraged to take over.
You want to take a tip cutting at approximately this point, including at least three or four leaves on the cutting. There is no need to take a significantly longer cutting than about five leaves, unless you want to make multiple cuttings of two or three nodes per cutting. Avoid taking off the whole cane on such a relatively young plant; that may slow down its progress.
By the way, this is a hybrid between Nepenthes bellii and N. aristolochioides, and is very adaptable to a range of temperatures (one is a warm growing species, and the other is from a cool climate).
For much more detailed instructions on stem propagation of Nepenthes, take a look at tis excellent article on the International Carnivorous Plant Society’s web page:
Propagating Nepenthes from stem cuttings