What's the acceptable amount of photos to post on Instagram?

[Chuck]

Most studies I've seen regarding effectiveness of social media outreach on instagram show that 1-2 photos per day, with as many as 4 per day (spread out through the day) is effective without negative impact.

Here's one of the most helpful discussions I've seen -- it's dated, but still quite relevant:
http://www.appdaptation.com/2011/07/instagram-growth/

There are no "hard" rules, and the instagram api docs do not specify an enforced upload limit, but it has been reported to be set to 7,500 images per day (so effectively non-existent).

Generally (and I feel this way myself), I think 1-2 photos per day (of GOOD, quality, interesting photos) is great. More than that just becomes noise.