Other times it will take several sessions to finish the song. And yet there are even times it will take months to complete a song. What matters most is if you are satisfied with the end result. Some of my favorite songs have taken months to complete. You never know.
Every song is different.
Most writers want to be prolific and continue to crank out killer songs. But I believe quality is more important than quantity. It is always good to keep writing. I may have to write 20 bad or mediocre songs to get to that one great song. I tend to think the best songs that really move people are the songs written from the heart and soul.