I just wrote an article on this topic. My research showed that the younger your customers the faster they expect a response, especially if they contact you via social media. Also, more people are using email to contact companies with questions or issues. Email was the first choice for the majority of consumers. 

But always respond, even if it is just a statement, either positive or negative. Let them know they are heard. Most people with a complaint will remain loyal to you if they feel you listened to them and cared.