The hallow spell is instantaneous. That means not only does it last forever, it can't be dispelled or disjuncted. The only way to get rid of it is to unhallow the area.

All of its benefits last forever except for the additional spell you can choose to tie to it. You can only pick one spell per hallowed area, and that spell stops functioning after one year. Then you can cast hallow again to renew it, or select a new spell to have for a year.