A "zombie" trademark is one that was once considered "dead" or abandoned, but has since been awakened from the dead (i.e. its trademark legal status is active and enforceable). There is a presumption given to anyone challenging a zombie mark that three years of non-use means the mark is no longer enforceable. This implies, that anything less than three years is given the presumption that the mark it still enforceable. What's more, even after three years of non-use, the trademark owner may be able to demonstrate its trademark is alive and well, it's just that the owner has the burden of proving there was an intention to continue using the trademark, which may not be easy to prove.
