Kitáb-i-Aqdas

He who hath divorced his wife may choose, upon the passing of each month, to remarry her when there is mutual affection and consent, so long as she hath not taken another husband ... unless, clearly, her circumstances change.   ¶68

Shoghi Effendi states, in a letter written on his behalf, that the intention of "the passing of each month" is not to impose a limitation, and that it is possible for a divorced couple to remarry at any time after their divorce, so long as neither party is currently married to another person.