The Wireless Association (CTIA) published Consumer Best Practices maintains that subscribers need to be contacted at least once every three months in order to remain certified for mass mobile communication.
This is another reason why it’s important to be specific in your opt-in message. The more time between correspondence, the more likely subscribers will question the origin of a message. Thus, if you are running a sweepstakes campaign and announcing winners after three weeks, say so. Make the user experience of your mobile campaigns as seamless and transparent as possible to earn maximum ROI.