When making these estimations, it's impossible to know with high accuracy exactly how long any activities take. That's not your goal. Rather than exactness, you should strive to set a worst-case bound on these estimates. The idea is for the group of experts to be able to say to each other, "We don't know exactly how long it would take, but we can all agree it's at least this much." Specifically, you should estimate a maximum reasonable time to consume your project output and minimum reasonable times for consumers to otherwise home-roll, use and maintain their own solutions.