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If you're talking about the Sendai region, they felt a massive offshore earthquake that lasted for more than two minutes. Automated systems in Japan alerted the nation that a major quake was occurring 10 seconds into the earthquake.

A little less than an hour later, the major drawback of water occurred along the coastline, and 1 hour and 15 minutes after the quake, television cameras captured the massive waves that wiped out entire regions.

So: From feeling quake (end of initial rupture): 1:15 From drawback of ocean water prior to tsunami: 20 minutes. Tsunamis come in multiple waves, each one building across the previous one, the entire event took well over an hour. During live coverage, I watched people in cars out driving the waves, and alas, some who were driving parallel to the face of the waves and debris who failed to get out of the way.