I would say most plasmas should take 30 to 40 seconds for the TV to start up, initialize and test the system, and then download a full frame of picture and audio - 30 seconds minimum for sure.
Once it buffered it's video/audio streams, it should not have any delay until you change to another channel, dumping it's buffers and reloading them with the new channel information.
All TVs take about 30 seconds from startup to picture displaying nowadays.
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Tips are greatly appreciated !