Hi Michael,
I don’t have my Pi set-up right now to time it but it’s is not instantaneous or quick, maybe upto minute or so. Raspian the Linux o/s takes a while to boot even without a GUI. That ROM running in the emulator would also greatly vary the emulator launch time. Basic early 1980s games such as Space Invaders would be much quicker than more complex 2000s titles such as Metal Slug. You can also overclock the Pi so that would improve things as would the quality of the SD card used.
This thread talks about a headless boot times in more detail.
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=29&t=25777