Good question!
Um, I usually compose when need arises. When some band or project needs a song, I'll just make a handful. Always found it the easiest aspect of playing to be make songs. That would mean maybe once a month or less. Sometimes I just ask the guys to shout out random chord names and make them work as a song, usually works just as well as anything else.
However, I'd say maybe ~six times a year I kind of spontaneously come up with a song that is a bit more complex or thought out, you know, home alone at night and watch TV and play the guitar, most situations like that a song just comes from somewhere, usually 2-3 parts for one at least, and then I figure out how does the rest of it go. I record it on my cell phone for later use...I have a bunch of those awaiting on my cell even now. In case I need a song somewhere.
When I get a song from like some dude without any vocal melody, i.e. just the base and am asked to make a vocal melody, I just hit record and sing whatever comes to mind. Maybe have to take a few stabs at it, but often the very first spontaneous melodies work the best.
With my Internet band, P-43, I take a lot longer with the melodies, used to actually compose them on a midi keyboard which no longer works, sadly...but with P-43 these days I'll first play the chorus over and over again and test different approaches, then verses, and then sing a demo of it which I may or may not send for approval by others, and then when I'm happy with the melody, the final take. Which kind of is a problem due to having so little time to do that stuff these days. But I have to spend the time because I don't want to make just regular heavy melody stuff - especially after three albums I feel I just have to find a little different approaches than just follow the normal scales over the chords.
I guess making vocal melodies is composing as well, so, maybe I then all together compose some 30 or so tunes per year, i.e. a couple each month. A little over one third of them end up on some demo I guess, the rest go to desk drawer or are simply forgotten later.
Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.