Well, it takes some weeks to learn how a config / material etc. works in ArmA.
But even, when you have a complete model, import and bugfixing can take some months (the RAH-66, which I imported from OFP, took me 3 months. The resulting quality is somwhere in between OFP and ArmA1, so it may take about 2 more months to make it A2-quality...).
So I calculate 3+ months for a tank/chopper import, about 1 month for an infantry weapon.