In the northern United States, wintered horses should be dewormed just prior to being turned out onto pastures in the spring. In this region, it is only necessary to suppress contamination past the beginning of July. After this time, there is insufficient grazing time left for significant pasture infectivity to develop. A final deworming is recommended when the horses are stabled for winter (in combination with a bot treatment).
In the southern United States, the timing is different. The high risk for infection occurs at the end of the summer. In spring-summer, it is too hot for eggs to develop into infective 3rd-stage larvae (even if there are lots of eggs). Risk of infection increases during fall - a result of newly deposited eggs which are now able to mature. Therefore, pasture contamination should be prevented by deworming just prior to the time pasture contamination will start occurring (September to October).
For more information, please read the article Equine Parasite Control.