18 month period? I'm not sure what you are referring to. The AW design and basic feature set is 24 months old right now. The Series 1 and 2 models were speed bumps and speed bump + GPS, respectively. The absence of rumors doesn't mean a true upgrade isn't near, it means there are no rumors. Apple can still keep secrets when it wants to. If Apple wants to grow AW its imperative to give Series 0 holdouts a reason to upgrade and new prospective owners a reason to be wowed over where the Series 0-2 did not. If it can get a significant # of Series 1 and 2 to upgrade that is gravy. But whatever Apple does or does not come up with will show us how serious they are about AW.
The first Gen Watch was released in April 2015, Series1/2 was 18 months after in September 2016. IF Apple releases another Watch this Fall, that's one year after the Series 1/2 update and it's likely another incremental update with LTE, which has been the main rumor (Aside from a different Touch display). There hasn't been a lack of rumors, there just has been very few.
And Watches generally don't change design all that often, and knowing how Apple isn't known to drastically revamp products on an annual basis, "Series 3" will likely be another minor update. Regardless, I think Apple is showing plenty of signs it's serious with the Watch through marketing and band refreshes. I don't think having a complete redisgn proves their serious, I think it indicates they are committed. Again, a major redesign doesn't seem likely this Fall. I believe 2018 will be the bigger changes to come.