I cured myself of vampirism and asked serana if she wanted to and she did but she wanted to do it alone and then meet me back up. So I was like ok. Then a day went by and I went back to morthal to see her she was in falions house so I waited twenty four hours and now I can't find her where is she or how long will she be gone

She goes back to the Dawnguard fort, she takes a few days to get there but she'll turn up.

She seems to dissapear to go do her own thing if you let her go. found her killing bandits in helgen a few days after i fineshed the dlc.

I believe she is at the place where you cured yourself. If not she has probably took a road back to the fort.