There are authorities who advise a woman not to immerse at all when her husband is out of town (or when she is out of town). However, the basic halacha is that immersion is permissible.

If there is no definite custom in your community or family to delay immersion in this situation, and certainly if there is any chance of an unexpected reunion earlier than planned, it is a good idea to immerse as scheduled.

During niddah, there are constraints on speech between husband and wife of a nature that may lead to intimacy. These constraints apply even when the couple is apart. If you were to immerse in a mikveh, becoming tehorah, these speech constraints would be lifted, and you could speak freely. It would still be critical to keep in mind, however, the prohibitions of hirhur, sexual fantasizing outside of the context of permitted relations, and of spilling seed, hotza'at zera levatalah.

Some communities have particular customs that apply if a woman immerses while her husband is away. In a community where such practices are customary, they would be appropriate. You could double check with your local rabbi where he stands on this issue. Otherwise, many halachic decisors, including our site's supervising rabbi, Rav Yehuda Henkin, deem such practices unnecessary.

Please write back with any further questions.