FANTASTIC question. In fact, I had recommended against creating one for the last company I was with because I felt it was too early.
I think it depends on the age of your community, why you are creating the program and what you hope to gain from it:
If it’s just to help increase posts or drum up some brand awareness, then earlier would be better. I think a year’s worth of data is reasonable. I’ve found that activity in communities changes season to season (and can vary amongst different communities) - and you’d want to see those patterns. At least a year’s worth of data also gives you an idea of who is committed.
If it’s to meet company goals/drive profit, I think you need more than a year of data to look at. You also need to compare it to the company’s data/analytics to see how they relate.
Based on your post, I don’t think a launching formal ambassador program is your answer here. I think that before you can get someone to volunteer you have to have established a solid reason for them to return. I’d shift my focus to creating regular programming, events, and other retention methods.
Once you start being able to identify potential ambassadors and begin developing relationships with them (and between them) that will help give them reasons to continue investing their activity. If you have a handful selected, you can use them to help you develop the program based on their feedback.
But whatever you decide, I think it’s more important to focus on starting SMALL and INFORMALLY to see how it develops on its own. If you don’t have any obvious candidates, don’t launch one. If you have some gut feelings of potential ambassadors, start developing relationships with them and BETWEEN them NOW.
Hope this helps!