Nothing specific about the numerical value of any particular Duty Awards for actions taken during a session or over the course of a series of missions is given in the Core Rules.
This is left to the GM to determine and relates to the speed with which the group would like to progress in Contribution Rank. The more frequently the group can raise their collective Duty to 100, the more often their Rank will increase. With that increase come greater responsibilities, recognition, notoriety, and Imperial resources dedicated to dealing with their activities. The tone and duration of the campaign desired by the group will need to be the deciding factor on how often, and to what degree Duty is rewarded.
Duty in play is described in Chapter 9: The Game Master under "Using Duty" on page 325 of Age of Rebellion. In that section, it reinforces the idea presented earlier that individual Duty may or may not play a role in any particular mission, but over time, the combined efforts of the group will see the characters contributing to the Rebellion in their own ways. Duty and Motivation are tied together, and the interplay of both of these 'pillars of character' will be involved in rewarding their attempts to act on behalf of the Alliance against the Empire. If they spot opportunities and find ways to act on them, their Duty will increase. This is also discussed briefly in the campaign development section, specifically in choosing the rewards carefully to suit what the group agreed to play (p339).
Core Rules Example
Numerical examples were not provided in the introductory mission, 'The Perlemian Haul,' in the core rules. Rather, an example of how to narrate the rewards was provided in an example on page 456 that those with the Duty (Personnel) would grow in renown in the Alliance as a result of rescuing captured Rebels. Those they freed are described as being sure to get the PCs' names, and then spread them among their contacts in the telling of the tale of their rescue.
This is important to note, as whatever numerical value you assign, this is to be a reflection of how the character is perceived as a contributor in that area. The group is working for the goals exemplified by the Rebel Alliance, and as a group their Duty will at times advance, and they will earn specific accolades. What are those to be, and what effect should this have on their position in the Alliance?
Not all missions need to end with a change in Duty. It should occur often enough, and opportunities for characters to engage with things which are tied to their Duty that it actually serves as a pillar of characterization and identity. The first step is establishing the need to take these Duties seriously and seek ways to fulfill them on the part of the characters. As those efforts take root, then the rewards can begin to be assigned.
Published Missions
In the Onslaught at Arda 1 scenario book, at the end of each chapter, a range of appropriate XP and Duty awards is provided with specific breakdowns of how they are earned for each 'Episode' laid out in the book. Generally speaking, the Duty awards for an Episode fall within a 1-10 range, with the opportunity for Bonus Duty if a specific aspect of the mission was successful, or if the group accomplished a session's triggered Duty.
Within the material for each Episode, sidebars describing opportunities to act on Duty are provided with specific rewards listed for completing them. These range from 2-5 and appear to increase with the amount of opposition or threat will be faced in the attempt. Locating equipment is a low reward (2), recruiting is a moderate reward (3), while a victory in combat may receive a higher reward (5).
The picture of rewarding action painted in this scenario is one of providing normal XP for each session, with the possibility for earning a Duty Bonus if an opportunity arises during that session. Once a phase (Episode) of the scenario is complete, additional XP will be awarded for accomplishing very specific things. Likewise, looking at the Duties and Motivations of the characters, Duty for the group will be boosted 1-10 points based on the state of things at that point in the scenario as a whole.