I run projects with companies that last for 3-6 months, delivering complete data warehouses (at least the first iteration, documentation and trained permanent employees). And that includes the time to start things up, get the resources, people and materials, etc.
18 months is longer than my longest project to date. I've had to fire someone from a project where he wasn't making a positive contribution in *week two*. Let alone 18 months. I really can't imagine that.
Maybe it's a bit like the people who claim you need years of experience with a certain toolset to become proficient. To which I reply: "so getting a Ph.D. in physics or CS is easier than learning the quirks of your tool? Get a better one." The same goes for your business. If it takes 18 months to learn the quirks, there's something wrong and perhaps you need to bring in someone with a fresh perspective for a second opinion.