Tasks that take few hours over a long time?

I need to be able to manage tasks that will be spread out over time. For example, something might get 20 hours over the course 10 months. I don’t want to make it a 10 month task because it’s important to track the actual 20 hours of effort. Anyone else had to do this, or have suggestions for how to enter and specify these kinds of tasks in OmniPlan?