I tend to get a rhythm, bass line, and melody lines in my head more or less all at once. Elapsed time= two minutes.
When I can get to the computer, I will plink up a working version of the parts, just slam things in, not too careful about editing or perfection. Then I'll make an mp3 of that. Time elapsed = about 45 minutes.
Then I will listen to that mp3 about 1,000 times. Not an exaggeration. I will add other parts in my head, and probably here come the lyrics as well. Time elapsed = 6 months.
Finally, I redo the computer tracks, make several iterations with improvements. "Improvements." I'll take version 1.0 up to version 17.0 or so. Time elapsed = several years.
At this rate I can put up maybe two songs a year on the Songs forum! Yea!!! But hey life happens and that's why there are those burners on the back.
I've got a killer song about George W Bush that is only 12 years old at the moment. Boy, wait until that one drops! Most likely, I will first.
cheers,
-Tom