Does anyone know if it is possible to go through a remission with idiopathic hypersomnia and if so how long does it usually last?

Idiopathic hypersomnia in its truest sense is 'probably' genetic and remissions followed by relapses do not occur. If there is an alternating sleepy then not sleepy period, one must consider a different diagnosis. Idiopathic means there is no other cause. IH can improve with age and go away. But it should not go away, then come back, then go away. Those patterns are more likely with chronic mono, thyroid disorders, changes in medications/stimulants such as coffee, tea, etc, subclinical or atypical depression or few other psych diagnoses (Bipolar, remitting cyclothymia, others), shift work changes, seasonal affective disorder, you name it, but not IH.