In response to a question from @BrailSays pointing out that the description of the Bag of Tricks didn't specify a duration or a limit to the number of creatures that it could create, D&D 5e rules designer and official rules expert Jeremy Crawford tweeted the following errataclarification on 09 Jan 2015:
The bag of tricks is missing its duration: 10 minutes, 1 animal at a time. We'll correct it.
In response to this question from @BrailSays:
Bag of Tricks...no duration listed, nor limit to how many things total, just per day. Can amass army of critters?
This has since been given an official errataan official errata, as explained in this answer by daze413 by daze413.