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Well, if the 2.33Ghz iMac that I sent back is anything to go by - it will be fast. The iMac took 20seconds to reach the desktop and be fully functional from the 'dong'.
 
Never actually measured it, but then again I rarely reboot or shutdown the machine. In any case, it's not very long, a guess would put it under your 20 secs.
 
It's going to vary from machine to machine. This is due to ram, hard drives and how many software programs you have installed. If someone tells you theirs launches in 20 seconds, that has very little meaning if their computer is running the stock configuration. If your machine is loaded with ram and software, it's going to take longer to boot.

Just be content with knowing that it's fast.
 
I have the default config'd Mac Pro, and it boots between 10-15 seconds. That's on the Mac OS side of course. Windows takes more like 25-35 seconds.
 
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