Okay, here is my experience with this.... Round worms are common in youngsters. Around 18 months of age they start building an immunity to them, roundworms are one of the only parasites that a healthy horse will begin to ward off on its own when it matures, as long as the numbers are kept in check. Older horses with a compromised system are at risk for picking up these parasites too.
I would wait two weeks and do the 5 day double dose of fenbendazole. The only reason I say wait two weeks is to make sure your horse does not get colicky while the worms die off. A bit of oil or milk of magnesia does not hurt to keep things moving, as well as soaking hay or adding warm water to the feed to make a mash. This ensures the horse is hydrated until they are passed. Often times the horse seems a little off....a bit drowsy, a little less energy than expected, a little bit of a droopy lip while standing around. After things seem like they are in check, then start the five day treatment. Be prepared to soak the feed and hay as a precaution then also. Glove up and check the feces. You will need to do this each time you worm until you don't see them anymore. Some horses don't shed eggs like others and it does not always show up on a fecal. I would reworm again one month after the five day course is up with another five day coarse as a precaution. The fenbendazole is one of the only coarses of action that kill all stages of roundworms. Young horses need to be wormed each month, regardless of a fecal. They are more succeptable. Bad news is that once you have roundworms in the environment you must be extremely vigilant of very young or older succeptable horses worming schedules. The eggs and parasites can remain active up to ten years in unused fields, longer in fields being used where they can continue to ru their life cycles. Cold temps do not kill them, some studies have shown composting and achieving high temps within the compost may have some effectiveness on killing the parasites.
Dealt with this this summer/fall with my GV colt. He is as healthy as can be now, but please be proactive in heading off possible complications that may come hand in hand with them. As my vet said, even when purchased from a good breeder with adequate care, it happens. A foal can spit out the wormer, a dose can be skipped, maybe they think every two months is frequent enough to worm a foal, but once roundworms are in the environment, they stay there for a long time. Glad I have no other young horses and have no intention if having anymore.
I wanted to add that others are correct, you will never have a completely parasite free horse. Roundworms and tapeworms can wreak havoc on a horses system if not kept in check. Don't know if you did any research on them,but there are some pretty nasty pics, especially involving colic/impacting surgery involving them.