Several years ago, a colleague emailed me, asking how to keep sponsors involved. My colleague was using company-mandated phase-gate lifecycle with long project durations (18-24 months).
I'd recommended providing a project dashboard and showing the sponsor progress. My colleague was stumped–the dashboard wasn't particularly helpful until they were in the testing phase and it was hard to show progress because they weren't using any formal iterations.
I explained about iterations, and suggested that the have something to show the sponsor every quarter at the sponsor's review. “Every quarter? That's way too fast!” was the inevitable response. I consulted with them a few years later and diagnosed the multi-week build times as a huge problem. (Jeff Nielsen of Digital Focus gave an intriguing talk about “The Psychology of Build Times” at Agile 2006 and will do another version at SD Best Practices.)
Iterations point out weaknesses in the project's infrastructure. If you can't do builds as often as you need them, your iterations can't succeed. If you can't show progress on a regular basis, you can't keep the sponsor involved. Weaknesses in a project's infrastructure breaks the project rhythm, leading to longer projects that deliver less than what the sponsor wanted.
So if you need to keep your sponsors involved in your projects, move to iterations of some variety. I prefer iterations of 2-4 weeks, but even quarterly iterations might work, as long as you have regular builds (at least nightly), so you can catch problems (in the project and in the process) early.