How Many Months Apart Should A Horse Be De Wormed?

Actually, many people de worm their horses 6 times/year (every two months) others 4 times per year (every three months) and others only 2 times (every six months) or yearly. Depends on your product that you use recommends... However;

www.thehorse.com states: "One researcher said 25% of a worm population must be resistant before you start seeing decline in the fecal egg counts (the poop tests you send off to see how many eggs are being shed by the parasites in a horse)  or a shorter return of high fecal egg counts. In other words, you deworm, the drug knocks down the adult parasites that are laying eggs at that moment, but the next generation of worms springs into action and egg counts return to pre-deworming levels (or higher) faster than they should."

I recommend you ask your trusted vet for his recommendation.