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 2:229 Divorce is twice. Then, either keep [her] in an acceptable manner or release [her] with good treatment. And it is not lawful for you to take anything of what you have given them unless both fear that they will not be able to keep [within] the limits of Allah . But if you fear that they will not keep [within] the limits of Allah, then there is no blame upon either of them concerning that by which she ransoms herself. These are the limits of Allah, so do not transgress them. And whoever transgresses the limits of Allah - it is those who are the wrongdoers.
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.