The legend and lore is that this challenge sprang from a conversation between professional songwriters about doing 50 pitch-able songs in 90 days. Writing a song a day or two is certainly acheivable for someone doing it 40-50 hours a week, like someone working full time for a publishing company. The irony is that the publishing company would own the songs and would probably not like them being posted on a publicly accessible website in raw form.
For me, I profess to be a songwriter but do not aspire to pitch songs, get cuts, or dive into any similar aspects of the music business with the notion that I could earn a living from it. I spend maybe a few hours a week between challenges, and usually up to 10 or the occasional 20 (like if there is a superskirmish weekend) during the period of a challenge. 2016 I finally achieved my personal 50/90 goal of 50+ solo demos (not counting collabs or chain-writing games like a morph, etc.) with words and music. I need to relisten but feel like I at least like half or more of them, and there is material to work with.
So for me, this year my goal will be to write and post 50 songs I like. I tried that in 2015 and ended up posting a lot of half-written not so well liked songs (that became zongs) in a mad rush at the end. Trying for 50 songs I like again this year with my recent approach of theme based writing. Not sure what I will do what the ones I don't like yet, maybe another late posting so the site gets song count credit, knowing the songs will likely be zongs.