My colleague, Jeff Howey and I respond to a question I got from a student in one of my classes:
"I would like to know how long should milestones typically be and how many sprints should we break it down to. We have a goal of what we want to achieve and a rough timeline but we don’t log too many feature tickets ahead of time thinking that the task might become stale or pollute the board with an everlasting list of things to do and most of the time we were just closing the tickets. As a result, I feel we become short-sighted and optimize for the current sprint but not for the milestone."
You can find out response here.