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As with any clinical condition, relapses are not uncommon. You may notice a relapse despite having had therapy during times when your mood changes i.e becomes dysphoric or depressed, during anxiety provoking situations, at times when there is some kind of a trigger in your environment ex: dealing with a person that may evoke unresolved/suppressed issues and feelings, etc.
DBT is generally the used therapeutic modality for treating BPD. You may already know that personality disorders are not per se "curable" and that the individual who suffers from such a condition would have to frequently monitor their mood, circumstances that may be acting as triggers, their emotional and mental reaction to these. If there is a co-existent disorder such as anxiety or depression, these too would have to be managed (medication is an option) along with receiving your treatment. If you find out that you're relapsing, it could be helpful to see a therapist for support at least for the time being.
As far as a specific frequency for relapse, that would depend from individual to individual (their triggers both external and internal, their coping skills and their world view). Alysha, please feel free to respond as needed prior to rating. Thanks!