The answer to this question is highly dependent upon the species of Nepenthes you
have, and that is something which you've failed to mention. Slow growing species, such
as N. villosa and N. lowii, can take two to three months to form pitchers. More rapidly growing
species, or hybrids, can take as little as a few weeks.
In order to assess whether your growing conditions are inadequate, or you are simply
a bit impatient, you should indicate how long you've had your plant, and what your
conditions are (i.e., temperature, light levels, relative humidity, photoperiod, etc).