I deworm my horse twice a year- once with a 'regular' dewormer (ivermectin, fenbendazole, or pyrantel pamaote) and once with one containing praziquantel for tapeworms. In between deworming I run a fecal egg count (FEC) to see if additional deworming is needed, but so far I've never needed any extra deworming.
Since your horse hasn't been dewormed in a while, you might want to start with a FEC to see what you're dealing with. If your horse has a heavy worm load and you kill them off all at once you run the risk of causing an impaction colic. Running regular FECs will also let you know if your horse is a high or low shedder of worm eggs and if your deworming program is working.
“The horse is a mirror to your soul. Sometimes you might not like what you see. Sometimes you will.” - Buck Brannaman
"Nothing forced can ever be beautiful." - Xenophon