P wave travelling through earth

Does it take 20mins for the P wave to travel right through the earth or only to the centre?

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If I get your question right, the time duration for a P or primary wave to travel through the earth during earthquake which result from a sudden release of pressure as a result gradual built up pressure within the crustal rocks, should be determined by the energy released at the origin of the wave and the medium the wave is traveling through(air, water or solid)Otherwise, all earthquake intensity readings on the Richter scale would have been the same, if it only takes 20mins of it to travel through the earth.