I try to do it when I finish a book, but found I sometimes don't until I am on the third book after. I wanted to do a blurb about how I liked the book, but found I haven't done that either. I plan on trying to catch up on that part, at least so I will have some kind of record for myself.
I have discovered that I am having fun not buying any new books and just going through all the freebies in my TBR pile. If I don't like it, I just delete it and don't record it. Surprisingly, I have found some good ones. I may even try to go longer than my goal of not purchasing for 6 months.