A phrase used to indicate something is incredibly slow, that you could take a nap by the time it finishes. It's usually used sarcastically and in exaggeration of things that are slower than they should be, but not literally long enough to take a nap (15 minutes or so).
"I might as well put it in park and take a nap by the time this frickin' light turns green."
"This computer is taking forever to boot up. I could probably go get a coffee, bang that secretary in accounting and take a nap by the time it's ready to go."
"You haven't finished yet? Damn, baby! I'm just gonna take a nap already. Wake me up when you're done."
"Hey, waitress, we're just gonna take a nap over here. Wake us up when you finally get around to taking our order."
Okay, I don't know if you people are mentally ill or just stupid but these definitions are way off. Taking a nap means to sleep for a bit if you feel tired but it's not quite time for you to go to bed.