Their goal is to cover all territories at least once a year. Congregations that can't do this can get assistance from the home office or surrounding congregations to cover their territories.
Of course, what they call 'cover' the territory is a lot different than it used to be. Lots of publishers knock so quietly that nobody would ever know they were there. Is there a No Trespassing sign anywhere on the premises? Skip the house! Nobody at home? Leave a tract and forget about it!
A lot of territory coverage (maybe 30 to 40%?) consists of Memorial/Convention invitation canvassing, where no real message is conveyed. Gone are the days when JW's prepared, rehearsed, and delivered a 4-5 minute sermon at the door which actually included Bible scriptures.
This generation of JW's seems more interested in making an appearance on Saturday morning, working a few houses, then hightailing it to McDonald's/Starbucks/Panera Bread for a lengthy break.