On a recent, late-evening flight from New York City to San Fransisco, I was seated on the right side of the plane. We were flying over lake Michigan - about 2 hours into the flight. When we were over the mid-west I looked out of the window and could see these continuous flashes in the sky. At first I thought it was lightning, however this continued for almost 30 minutes or so and seemed to be silent too. What is this phenomenon?
You were most likely seeing lightning (to be sure, a date/time and your location would be necessary). Lightning can be visible from quite a distance and storms can be incredibly active with lightning within the clouds and from cloud-to-cloud. Given a large distance and being inside a (relatively) loud airplane cabin it is no surprise that you couldn't hear the thunder associated with the lightning flashes.
While I wouldn't rule out that you were witnessing distant lightning lighting up the clouds below, or at some distance anyhow, there is also the possibility of the flashes being upper-atmospheric lightning such as Sprites. The half hour duration of the phenomena seems quite a long duration if we assume a plane in flight would cover somewhere about 300 miles in this time. So another possibility could be St. Elmo's fire and another possibility would be wing condensation flashes over the wings, reflecting the light from inside the plane. A youtube video of the effect at take off in daylight. Or else possibly a cloud situation that reflected back the plane's own flashing exterior lights for this portion of the journey.