Depends on size, speed and how 'central' it would hit. Possibly also on composition (if it's very 'loose' then there's less effort to disperse it). All of these would determine how difficult it is to deflect...or whether it's possible at all. Beyond a certain size there's just nothing we can do.

But the idea is to detect them early and then have several years (5+) to stage a deflection mission. The further out you deflect the less change in its path is needed.