Oh, about as long as you'll let him. Meth generally clears the system within a week, pot a lot longer and alcohol in about 6-8 hours. Depending on the amounts he was doing it is probably best to get him in front of a doctor. The real question is what is he going to do NEXT. Drying out, or detoxing doesn't last nearly as long as the cravings and psycological effects. This withdrawal easily and frequently leads to starting all over again, despite the promises he will be making to you. I strongly suggest some form of residential treatment followed by sober living for him. I suggest even stronger that you visit our Nar-Anon boards for ways to take care of you, no matter if he stays sober or not. If you need referrals for help, please let us know the state and county you are in. And do take care of you...
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Be well, and have a great 24...