There were articles posted about anthrax that stated that it could be expected to survive for "months" on the isolated islands where it was tested. It seems to be quite hardy.
On the other hand, I do wonder about the dispersion.
Anthrax, in its weaponized form, unlike most other bio and chem weapons, is indeed quite stable. It may be viable for years, not just months.
There are websites you can go to which plot out the area an anthrax release could be expected to effect from a single bomb. It dissipates within miles downwind, much less thousands of miles.