Yep, how often and when to deworm very much depends on a bunch of factors including housing/pasture situation, age, geographic location, etc. Deworming every 6 weeks isn't a good program because you are deworming again at the wrong time after the last drugs used. You are either allowing egg shedding to continue for a couple of weeks before the next deworming or deworming too soon when there is nothing for at least 1/2 of the common deworming drugs to treat. Strategic deworming should be practiced so that you only deworm when necessary to treat the parasites you are dealing with. Some horses can get by with deworming only twice a year---others may need a more stringent deworming program but again every 6 weeks isn't appropriate for any of the drugs on the market.
And in PA there isn't any reason to deworm in the middle of winter. In the northern US the reinfection rates in winter are at their lowest. Deworming should be done after the cold weather begins and then not be done again until spring. In spring, all horses should get ivermectin or moxidectin (generally along with praziquantel) to treat for bots (and tapeworms) to help break the life cycle and keep from recontaminating the horse's environment with these parasites.
Licensed Veterinary Technician