The following graphic takes returns from 422 of the current S&P 500 constituents over different periods from 2002 (their shared data start date). Returns are split into 5% buckets. What we see is that you are just as likely to see a 15-20% return over a 3 month period as you are over a 7 month period.
To us, this demonstrates the importance of removing time from the momentum equation; the term structure of returns is rough, volatile, and ever changing. By constraining ourselves to evaluate momentum over a fixed time horizon restricts the opportunities we can pursue.
For another look at momentum across asset classes for different time periods, see this post.