D&D 5e designer and official rules expert Jeremy Crawford tweeted the following errata 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?