As I previously said, the target of 40 pieces equates to learning/playing one piece every 9 days. You could just choose a single piece to play for 10 or 20 mins each day, then move onto the second piece on day 10, and so on. Or you could add a new piece every 3 or 4 days, until you are working on 3 pieces every day, then each time you finish a piece you could start another, so that you constantly have 3 pieces which are in different stages of progress. This way you might find that for the first week or so you don't finish any pieces, but then you could find yourself completing a piece roughly every 3 days. It depends how much time you want to spend on this challenge. I think if the pieces are easy enough and you don't feel the need to polish them to perfection, this would be very doable, unless you are also working on complex stretch pieces for exams/recitals, in which case you might not want to commit extra practice time to this challenge.
"Don't let's ask for the moon, we have the stars." (Final line from Now,Voyager, 1942)