If a person does some bidah in his life and he goes to hell for it. Will he remain forever in hell or he would come out of the hell.
This depends on his innovation.
If he did not associate partners with Allah, he will be punished for his sins to purify him before entering him into paradise, or he may be forgiven, this is all in Allah’s hands. The basic ruling is that a Muslim who did not associate partners with Allah will never be in hell forever.