When I decide to write a song, it can take between an hour to a week. The songwriting process can be a lot of fun if you come ready to write. For me I always try to get in a mood of writing either by recalling memories and feelings or doing something that makes me feel a certain way. For example, if I take a walk in the nature it is more likely that I will want to write a ballad or inspirational song, if I decided to dance it is more likely I would want to write an upbeat dance song.
Some people prefer writing lyrics before the melody but I like to do the opposite, but there’s exceptions (which will be released soon).
My co-writer is my guitar. I love writing songs on my guitar because the guitar is such a great accompanist. The only thing with writing songs on guitar they turn to be Rock songs (which I LOVE) but my final product (for now) is in POP genre.
There are many things to watch when writing the song. Rhyme scheme, structure of the song, bpm and so on, but the most important thing is that the song makes sense to you as the creator and to your audience.
Also, you have to think what audience you're trying to aim for and what is the message you want to deliver with your song.
All in all, the most important thing is to have FUN! :)
