When they complete a mission, or goal, or just do something nice to aid the rebels.

How much Duty should the group acquire each time, to push their standing up to 100 to gain a reward or recognition of some sort?

Is it the same as xp, 5-20 per session/mission? Are there guidelines somewhere I'm missing?

I'm currently away from my book, but there are guidelines, though, as with everything in this system, it is very flexible. For example, my group meets fairly rarely, so I give them each 15 Duty per session. That way, after two sessions, they level up and get access to higher levels of Alliance command or trade their Duty for a sick new ride/sweet new firepower.

I spent ages looking for an answer to this question! Apparently the devs talk about it in a podcast somewhere (Order 66 I think) and they said it should be enough to go up a contribution level every 3-4 sessions... so 25-33 in total for the group per session.

What I settled on was 5 per character per session assuming they do literally anything to further the Republic, and an additional ten per character that furthers their specific duty. Since I have 5 players, this results in ~35 total duty per session.

This used to drive me crazy until I found out that there was a section about this in the Beta that somehow missed being added to the final version. This is directly from page 205 of the Beta book:

AWARDING DUTY

Player Character earn Duty value through achieving goals relating to their Duty or making choices motivated by their·Duty. The GM can grant this Duty to PCs immediately upon them achieving their goals, or at the end of the session. How much Duty a GM awards depends on the campaign he is running and the magnitude of the accomplishment; though it is typically a value between 1 and 10.

From one of the Order 66 podcasts I believe Sam Stewart said that a player who achieves his specific Duty should get another 1 to 5 on top of this amount.

Obviously, this probably needs to be tweaked if you have particularly large or small group or if you play very often or very rarely.

Oddly enough I had only just gotten a chance to listen to this episode. It's episode 39, and was a specific question he was answering. Probably a good section to listen to overall as well. It's right at the beginning of the episode so it's easy enough to listen to and "dump" the rest of the ep.

Episode 39.

Topic 1 Duty (16:00)
Q1 Explanation on awarding duty (16:30)
Q2 chain of command within the party and within the rebellion (27:30)
Q3 Blending Obligation/Duty/Morality (32:15)

Topic 2 New Species careers and specializations (34:00)
Q4 Resistance of Bellow at engaged (35:25)
Q5 Commodore/Commander design process (37:25)
* Mechanics and solid repairs (42:30)
Q6 Spy has no Ranged (light), Infiltrator has no brawl (44:00).
Q7 Andy’s favorite species/specialization combo (48:00)
Q8 More species/specialization or cross-specialization combos (49:45)
Q9 Fusion Cutter (53:00)

Topic 3 Starships (56:15)
Q10 Chase mechanics (58:30)
Q11 Temple class freighter backstory (1:01:25)
Q13 Why only 1 silhouette 6 ships (1:02:55)
Q14 Sentinel Shuttle vs Lambda stats (1:05:15)
Q15 Tie Defender reduced speed (1:06:40)
Q16 Attempting an impossible task and heavy tractor quality (1:09:45)
Q17 No bulk is taken into account of tractors (1:11:30)
Q18 A-Wing strain threshold (1:13:20)

Topic 4 The Force (1:15:45)
Q19 Force rating accessibility (1:15:50)
Q20 Going from Age of Rebellion and Force and Destiny (1:18:15)