Most heartworm meds are given monthly just for that reason.
It is easier to remember. In other words, the company gets more money this way.
Not all drugs work the same though and there are some that you probably can't wait 2-3 months to give. If your dog does get heartworms, some companies will pay for treatment if you follow their protocol exactly like Heartgard. These drugs only kill certain stages of larvae too, so it is important that your vet is in agreement and helping you come up with a different schedule. I am going to be talking to Ellie's vet about the grace period of Interceptor and I may stop giving it monthly. I think Heartgard's is said to be 15 days.
By the way, there are a lot of reports about Revolution failing and dogs getting heartworms anyway while on it...