Halacha allows women to light the Shabbat candles without accepting the onset of Shabbat. For example, if a woman must drive somewhere before Shabbat and will not return home before Shabbat begins, she may light the Shabbat candles before she leaves and stipulate that she does not yet accept the onset of Shabbat. She then accepts Shabbat later, a few minutes before sundown.
However, the Shulhan Aruch writes (263:4; listen to audio for precise citation) that when a woman lights the Shabbat candles early, rather than shortly before sundown, she must accept the onset of Shabbat. Halacha allows a woman to light Shabbat candles as early as Pelag Ha'minha which occurs on the average 75 minutes before sundown, depending on the season but she must accept Shabbat at that point, because otherwise it is not evident that she lights specifically for the honor of Shabbat. This Halacha gives rise to the question of how early before sundown a woman may light Shabbat candles without accepting Shabbat. Said differently, at which point on Friday afternoon does it become clear that the candles are lit specifically for the honor of Shabbat, such that a woman may light without accepting Shabbat?
Hacham Yishak Yosef (She'erit Yosef, vol. 3, p. 383; listen to audio for precise citation) rules that a woman may light candles without accepting Shabbat as early as a half-hour before sundown; according to other authorities, she may do so as early as forty minutes before sundown. Thus, for example, if sundown on a summer day occurs at 8:30 PM, a woman who lights candles before 7:50 PM on Friday must accept the onset of Shabbat. If she wishes to perform Melacha (activity forbidden on Shabbat) after lighting candles, then she must light candles no earlier than 7:50 PM.
Summary: A woman may light Shabbat candles on Friday afternoon as early as Pelag Ha'minha approximately 75 minutes before sundown, depending on the season. She must accept Shabbat at the time of lighting unless she lights within thirty minutes or, according to others, forty minutes before sundown, in which case she may stipulate at the time of lighting that she does not yet accept the onset of Shabbat.