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It in dependent upon the environment in which the car is used, as well as its use. I have seen some with garage queens apply one coat annually. Other cars sitting outside 24/7 might need reapplication every 3-4 months. You could compound that by the durability of the sealant himself.
Now that said ... some actually like to apply waxes and sealants more often for that "just waxed" look. In these cases you may want to consider a paint cleansing lotion to remove the old sealant prior to application. In these instances, you may actually find that detail sprays and spray sealants will offer that look and you can apply them over the traditional sealant after a wash session.