There are a few things that can trigger chasing - change in temperature, heavier feedings, even change of weather if the fish are kept outside. Sometimes its the simple introduction of a new fish, or a new home for the fish alltogether, that triggers it. Its just the time of year, and "spring is in the air"....

How long after chasing? Well, that depends on the fish, and if its a young one or an mature one, and if the female's eggs are ready. It ends with the female expelling her eggs, and the male fertilizing them with milt.
Try lowering the tank temperatures a little bit, 1-2 degrees are normally enough, that should stop them. Lower temperatures tricks them into the feeling of cooler season, and thats usually not the time they are spawning. If nothing else helps, separating them might be your only chance - putting a divider in the tank, male on one side, female on the other....