Shush! is a 2 year old app, that will enable you to do just that. After certain period of time, for example 3 hours, the app will remind you to turn your ringer on and you can decide whether to turn it on, or to continue in silent. The app also will enable you to customize the colors and the layout.
So, that's it. The app is free, simple, useful if you are too often in meetings and lectures, it doesn't take too much space, so why not have it on your phone? You may check another app with similar features called Silence, that was already featured in our Best apps for students that will improve your college life list. However, Shush! has many more reviews and better rating.
Google Play Download Link: SHUSH!