Just would like to know how long u have do u have to stay in a drug rehab center for?

hi all i was just what to know how long u have to stay in drug rehab center for as my boy friend is meant to be in 1 that what he has told his mothere i have not heard form him in 5 week so i dont know what is going i just know what his mother has told me and i dont know what to do. i love him more them i can say and i dont know how to find him he will not tell his mother were he is and i have just not heard from him so i just dont know what to do so if u can hep me that will be good but if not thank u bec