If it was the third divorce then the couple can not remarry unless the woman marries another man and is genuinely divorced or widowed in the second marriage, and no passage of time will make them lawful otherwise.
Quran 2:230 And if he has divorced her [for the third time], then she is not lawful to him afterward until [after] she marries a husband other than him. And if the latter husband divorces her [or dies], there is no blame upon the woman and her former husband for returning to each other if they think that they can keep [within] the limits of Allah . These are the limits of Allah, which He makes clear to a people who know.
If it was the second or first divorce then they can remarry:
Quran 65:2 And when they have [nearly] fulfilled their term, either retain them according to acceptable terms or part with them according to acceptable terms. And bring to witness two just men from among you and establish the testimony for [the acceptance of] Allah . That is instructed to whoever should believe in Allah and the Last day. And whoever fears Allah - He will make for him a way out.