How long does seperation anxiety last?

My daughter will be 8 months on Feb. 7th and for the last 3 weeks bedtime has been a hassle from the seperation anxiety its not so bad during the day bc i am always with her but at bed she usaully is a great sleeper knows to go to bed but now she wakes and as soon as im out of the room she starts crying then i walk back in and shes okay even if she cant see me she just knows im there and i walk out.. crying again. I know its natural but has anyone dealt with this and know how to help it at bedtime and how long this lasts?im really getting stressed out:(