When to deworm and what with depends on several factors--
Local environmental conditions
How the horse is kept (stall, dry lot, pasture, boarding pasture, etc)
If pastured with other horses, are they on a regular deworming program
The environment where you are plays a big role in your deworming program because the weather plays a major role in how much of a parasite load there is on your pasture. In the summer months, the parasite load on pastures drops in the very hot deep south, in the desert parasite load is always pretty low, in the far north parasite load on pastures is very low (and horses aren't really getting much pasture time). Age is critical because very young and very old horses are more susceptible to intestinal parasites so they require more careful deworming. And you are looking at a different parasites as the main threat in very young horses than you are in adult horses and you have to be considerate of what dewormers are showing problems with those resistance in those parasites.
Management determines how much exposure horses get to infective parasite larva--stalled horses have much less exposure to parasites than horses that are pastured 24/7, etc. So, there is more than just going with the standard 6-8 week rotating dewormers.
Rotating dewormers is an old plan that was necessary back when the drugs we had available where not broad spectrum--one might get ascarids, another strongyles, etc--so rotation was done so that you were treating for all the different types of parasites. Then it was continued once we got better drugs available for deworming even though it's not necessary to be able to target the different parasites but in the hopes of preventing the parasites becoming resistant to the drugs. However, this has not been the case and with the situation we are now in there is very little "room" for rotating drugs (which in fact means rotating classes of drugs not just brands or chemicals) because the benzimendizole class (fenbendazole-Safeguard, Panacur, etc) is of very little use because of resistance issues in 95% of the world, and the pyrantel paste (Strongid, Equicide, Rotectin, Strongyle Care) has resistance issues in around 50% of the areas where it is used. The avermectins (moxidectin and ivermectin) are the only class of dewormer that doesn't currently have any resistance issues in strongyles which is the main issue in adult horses (though ivermectin resistant ascarids have been seen in certain areas and this is an issue for foals).
So, rotation options are limited at best and it's really not the rotation that is most important for preventing parasite resistance but appropriate dosing and treatment intervals.
You need to be SURE you are not under dosing your horses so always use a weight tape or measure your horse and calculate his weight. In studies even many vets were way off on weight estimations and they have the benefit of spending a few years working in situations where they get to walk horses onto scales daily to see what 900 lbs LOOKS LIKE.
Weight calculation information:
Body Weight Estimation of Horses KG calculation shown as well as a chart based upon heart girth measure (remember, the heart girth only assessment may be off by as much as 200 lbs just like weight tapes)
Horse Weight: Estimate It Easily How to calculate in pounds
As for appropriate dosing intervals, you want to treat according to the egg reappearance period so that you are dosing to prevent continual recontamination of your pastures. This method means that you will be cutting back on your horse's risk of parasite infestation significantly after a year appropriate dewormings because you will have cut the number of parasite larva on your pastures. It also means that you won't be deworming at a time when there are no parasites in the system that will be susceptible to the product you are using next which happens if you dose too soon after ivermectin or moxidectin with pyrantel or fenbendazole because at normal doses these two chemicals only kill adults in the GI tract and if you've dewormed with moxidectin last 8 weeks ago there aren't adults there to kill and you've missed the migrating larva which will then start shedding eggs 4 weeks after you treat with either of these drugs and shed for the 4 weeks until you deworm again. Or if you dewormed with ivermectin 6 weeks ago and treat with either pyrantel or fenbendazole you will start seeing egg shedding into your pasture in 2 weeks because you didn't have adults in the GI tract when you dewormed last but they will be there in a couple of weeks. Or in the case where you actually do have adults for the pyrantel or fenbendazole (which again is not likely to be working) to affect because it's been 8 weeks since your last dose of ivermectin you will again have parasite ova being shed in 4 weeks because that is how long after the use of either of those drugs you start seeing adult egg shedding parasites in the GI tract. So deworming based upon Egg Reappearance Periods, you would deworm and then deworm again based upon what drug you used last--4 weeks later for pyrantel (or Fenbendazole, though the next day would probably be more useful LOL), 8 weeks after ivermectin or 12 weeks after moxidectin. In this manner, you can significantly reduce the parasite load on your pastures in a year's time.
If you want to tell me a bit more about your horse, management conditions and weather where you are, I can help you figure out a decent deworming program. But it's really best to discuss this with your vet as well.
For a really good read on deworming, check out http://www.thehorse.com/ViewArticle....7317#parasites