Fifty times is a doable chart for anything that needs to be done in a week - it's only 7 times per day plus one.  I will sometimes have my students who are struggling with review fill out a 50 times chart for review that week.  Seven review songs per day plus one.  If they miss a day it ends up being more, but they are getting it done.  A reward - even a small one that doesn't cost any money - such as a day at the park - is perfect for a 50 times chart.