OmniFocus allows you to assign estimated durations to actions, action groups, and projects. Estimating how long something will take gives it more definition and helps ensure you don’t take on too much work all at once.
This plug-in calculates the total time (in minutes) of everything you select in the outline. For example, if you have a Today perspective that shows you everything you’d like to get done today, you could switch to this perspective, select everything in the outline, and use this plug-in to calculate the total time these tasks will take.
The Estimate Total Time plug-in provides similar functionality.