How long does rehab usually last for people with eating disorders?

Well my gf just told me she has bulimia and is going to a rehab type thing and said she doesn't know how long she'll be gone she's guessing maybe 3 months and she can't take her phone or have any type of contact like that while she's there so my question is how long does it usually last and is it normal that you're not allowed to take your phone with you? Any info would be helpful thanks