Worming with Fenbendazole can be done at any time of year that worms are suspected, but as a routine worming I do it in February and September. This can obviously vary from region to region, depending on when your Vet advises is the best time.
Five day Fenbendazole clears out encysted small strongyles more efficiently than any other wormer- Quest ( Moxidectin) claims to get then but, in fact in the small print only gets 88% of them, and in a single dose.
As we all know Moxidectin by any name is a no go area for Minis.
Fenbendazole should be administered at 1ml per 13 kg (28lbs) [this is what is recommended on the packaging, not my guesswork!! BUT in the US a double dose is recommended] and is easiest in it's liquid, generic form for accurate dosing BUT overdosing with Fenbendazole is only an economic issue- you would have to drown the horse in it to actually harm it!!!
Fenbendazole is not a particularly good wormer in a single dose and I only use it this way as a first wormer at ten days, for foals, or if the animal is debilitated by worms and needs a gentle clear out of some, but not all of them.
After the five day course I wait ten days (life cycle of most worms) and then worm with a combo of Ivermectin and Praziquantel (tapeworms)