Necrotic twigs, shriveled buds, shriveling and scaly bark. All in a year or two usually. Total tree death from the first outward signs was usually not more than two years. Most times I would see the stressed tree in Spring or Summer, and the next year it was dead. The actual process was probably longer, I'm just going by what I could see.
When it was this obvious I would never consider planting in that site. Moving planting locations always occurred when there was no visible history of a gas leak, and the first sign was the smell upon digging or grinding an old stump. We had contractors prep the sites before our volunteers would go in to plant, so many times we would get an early warning before the actual day of planting, which helped.
-Tom