D&D 5e designer Jeremy Crawford (@JeremyECrawford) answered the following tweet on January 9, 2015:
@JeremyECrawford @mikemearls Bag of Tricks..no duration listed, nor limit to how many things total, just per day. Can amass army of critters?
Crawford wrote:
@BrailSays The bag of tricks is missing its duration: 10 minutes, 1 animal at a time. We'll correct it. @mikemearls