I personally don't like to leave my compters on when I'm not using them. My Windows machine often gets left on, simply because it's a drag waiting for it to boot (a couple minutes), though that's nothing compared to my computer at work, which I NEVER turn off, because it can take 15 minutes...
My last kernel message has a timestamp of 5.4 seconds. It takes about a second each to get through BIOS and bootloader. The rest of the time probably gets taken up with loading the kernel from disk, and starting various vital extensions (wireless drivers, xvesa, flwm, ...)
In total, it's about 12 seconds from pushing the ON button to looking at flwm.