Dr. Faiza,
B12 commonly goes on for quite a while. I have had kids the methyl-b12 upwards of 2 years in some cases, and few longer. On average though it is about 1 year.
The problem with Methyl-B12 these days is getting it for more than a month or two. The methyl-B12 has always lost some potency after 90 days or so, but the pharmacies here in the US are under increased scrutiny and pressure to only release a short supply at a time. A newer option called a lyophilized vial allows for a longer supply, more so than the pre-filled syringes.
Contact College Pharmacy (in the State of Colorado) at http://www.collegepharmacy.com as I understand they still take prescriptions from outside the US doctors. Let me know what they say and I have had others get things from them, and they can too prepare Methyl-B12.
Dr. Woeller