How can I see the estimated total duration of the project on open project
I can only see estimate duration for each task
This feature is not supported directly, yet. This is an OpenProject dev writing.
You can work around this for individual projects by getting the work package with the earliest start date (sort by start date asc) and then getting the work package with the latest finish date (sort by finish date desc). The difference between the two could then roughly be considered the duration.
Not saying this is a good solution, it surely isn't. But it might be a viable work around for a single work package.
If it needs to be supported for all projects of an OpenProject instance, another option could be to have one work package that represents the project. All the work packages within the project would then have to be children and descendants of that "project"-work package. Users would have to remember to set that parent.
This is probably the way I would choose as it offers additional benefits, e.g. being able to see the project displayed in a Gantt-chart.