If you're sitting down to write a decent song, it'll take you less than four hours. On the other hand, if you're having trouble with a song you don't have. Fortunately, you'll learn a lot of strategies to overcome the effort of writing your song. If it takes you a long time to write the song due to problems with the lyrics, a little preparation can save you a lot of time.
Just like a painter has pre-mixed his palette of paints before putting the brush on the canvas, a composer must have the elements in place in the song so that it doesn't take so long to finish a song. Another fun chord progression you can use if you want to write a song quickly is the 12-bar blues progression. The main point is that it is possible to write the music for a song in a week in a consistent way, but a complicated guitar solo can make it unattainable. If you want to write a song and you don't know what the chords should be, it doesn't hurt to start with a set of chords that have been proven to work well together.