Good question Roger, I do a chain-link that is about 1/4 mile long every other mow. It is deep on the back side. I only do the side facing the pond area that I mow. This rough fence line always looks bad but this is what the owner wants. It takes a LOT of line when trying to get it to look decent. .095 line on a lesser power weed whip seems to make the line last the best.
Not knowing what type of fence, either way it would be a goal to get to is to be able to walk at a slow or moderate speed and still get a good looking job. More than likely this would be possable with Round-Up to help keep the big thick weeds and tree start ups knocked down. A moderate walking speed should not take 64 man hours. Some one is milking this job.