Here are a few time saving tips:
-Schedule tweets ahead of time using HootSuite. When you do have time to sit down, find topics of interest and schedule the tweets to go out during the week.
-Apps are a time saver. I use the HootSuite mobile app to get real-time push notifications so I can reply and interact when I have a minute to look at my phone. Google has the Google Reader app that can organize your Google Alerts. You can tweet the alerts you find relevant right from your phone. You can also schedule these tweets.
-While your computer is starting up, take a minute to look at tweets on your phone and retweet anything you find compeling.
It isn't about tweeting a crazy amount, it is about tweeting regularly and tweeting relevant and interesting content. Can't reply back and interact right on the spot? Do it when you have a second, your followers will appreciate that you still acknowledged them for sharing your content!