> We've got a backlog that will easily take us a year to complete.
To me, this is actually a great argument in favour of 2- to 4-week sprints. Long backlogs are psychologically demotivating. Finishing a task only to find that the backlog is just as long, or -- worse, and more likely -- longer than when you started is just so depressing.
Sprints give a small, subtle, but real psychological boost from routinely completing achievable workloads. It's when you overload sprints and can't possibly complete them that they start to break down. From a psychological perspective, you should usually complete all the tasks in a sprint. Not always -- but most of the time.
Of course, people who advocate for 6- to 8-week sprints also get this benefit, but, from my perspective, there's too much chance of having to change your plans partway through. The benefit of a shorter cycle is that it's easier to pivot everyone to work on something new. By way of example, my team at my previous employer was building a new service, and we discovered about halfway through that we had a design flaw that would prevent us from surfacing the level of detail we wanted for API errors. On 2-week sprints, we were able to use half of the next sprint to change our error handling across our entire code base. Sure, we could have done that anyway with longer sprints -- but then why bother having concrete plans at all, when you know they're likely to change?