It depends on the results you are looking to achieve. My typical response is 15-20 minutes in the morning and then the same amount of time in the afternoon.
Spend your time in the morning setting up your day. Find articles from your favorite bloggers to retweet or share across any of your social networks — respond and thank retweets and shares, and schedule your posts throughout the day. Then come back at the end of the day to answer questions and thank those that have shared your content.