Over the years, I've developed a bit of insight into the matter of when to repot.
In my humble opinion, it varies greatly from species to species. However, a
few general rules may be in order:
First, if you have a plant which was formerly small, but which has increased in size
markedly in a reasonably short period of time (say 6 months to a year), and which
is no longer pitchering, then it is entirely possible that the absence of new pitchers
is due to the restriction which your too-small pot is placing upon the ability of your
plant to obtain the moisture it needs for this task. I've observed this phenomenon
many times, and can now catch it pretty quickly.
Secondly, and this is really more applicable to lowland plants, if your plant begins to
droop a bit at the growing tip, even though you've watered it recently, then this is a
sure sign that your plant is likely rootbound. In extreme cases, entire plants can go
flat in hot weather pretty rapidly. Once saw this happen to a bical of mine. Immediate
inspection of the roots revealed that the plant was entirely rootbound.
Finally, if you can observe roots sticking up, either from the top of the pot (at its sides)
or from holes in the bottom, then the answer is clear. I've seen mirabilis put out
large mats of roots from the bottom of their pots, even when the pots have been
sitting in saucers of water (which is not something I recommend for many species other
than mirabilis, which prefers this, BTW).
Ultimately, however, it's been my experience that most Nepenthes can sit in their compost
for a long time. Usually, I repot because of the reason I outlined first: simply because they've
gotten too big, and because I've found that larger pots mean larger root networks, and
ultimately, larger pitchers.