One should not light candles, or do any other melacha, once it is two minutes to shkia (due to the obligation of tosefes shabbos, which, according to Rav Moshe Feinstein zatzal as well as other leading poskim, requires that one start Shabbos [at least] two minutes before shkia.
In a case of extreme and dire need, one may continue to do melacha until one second prior to sunset, since there are opinions that there is no specific amount of time that needs to be added to fulfill the mitzvah of tosefes shabbos. See Orchos Rabbeinu vol. 1, p. 105, that in a case of need, one can light Shabbos candles until sunset. If one is relying on this (perhaps in the winter months), extreme care must be taken to finish lighting and to drop the match from one’s hand at least one second prior to shkia. If a woman does not have enough time to light her customary number of candles prior to shkia, she should light as many of them as she can (or just one, if necessary) and drop the match from her hand at least one second prior to shkia. Rabbi D. Ribiat decries those women who cannot stand the thought of missing candle-lighting, and sometimes they light even after sunset, which is a safek issur kores.
Bottom line: One should make every effort to light candles 18 (preferably 20) minutes before shkia. This shows the proper respect to the Shabbos Queen.
If a woman is preparing for immersion, and she needs the extra time, according to Rabbi B. Forst shlit”a in his sefer on the laws of niddah, she should complete all melachos and light by no later than 5 minutes prior to shkia.