We are paying a subscription every 9 to 10 months. !
It is generally once per year, though the exact date fluctuates. They just do it when "it makes sense" based on finishing up projects from the "old version" before starting new big changes for the new version.
FWIW, the first builds of MC17 were released in September 2011. So we're just a smidge shy of 1 year. Generally, expect that when active development of the old version slows down (as it has here for the past month or two) and new builds are mostly minor tweaks and bug fixes, that a new version is coming soon.
However, I disagree with the characterization of their model as "subscription". A subscription software licensing model assumes that if you stop paying for the subscription, you lose access to ALL of the versions you ever had, new or old. JRiver does NOT do that.
MC17 will still be just as good in six months as it was for you two weeks ago, and it is likely that they'll continue to release new builds here-and-there with minor bug fixes for the next 2-6 months or so (until it is "done"). It will always keep working, and for the next few months, will probably be much more stable and less troublesome than using MC18 would be. It is not a subscription. It is just a yearly update cycle, like many other applications have.
But, yes, to review... There will probably be a MC19 released sometime next Summer/Fall. When they do that, they'll offer an "early-bird upgraders discount". You can decide when you want to take the plunge. However, if you've waited on MC18 until July or so of next year, and you look at Interact and see that the current build is 9 days old and the one before that lasted two weeks?
That might seem to you to indicate "it is stable, this is a good time to buy". But what it REALLY means is MC19 is coming soon.