My own anecdotal experience with this is in 100% of cases where people where to constantly change passwords they either added a digit to the password or wrote the password on a sticky note somewhere in plain site of the computer.
The most on-point data comes from a study published in 2010 by researchers from the University of North Carolina at Chapel Hill. The researchers obtained the cryptographic hashes to 10,000 expired accounts that once belonged to university employees, faculty, or students who had been required to change their passcodes every three months. Researchers received data not only for the last password used but also for passwords that had been changed over time.
By studying the data, the researchers identified common techniques account holders used when they were required to change passwords. A password like “tarheels#1”, for instance (excluding the quotation marks) frequently became “tArheels#1” after the first change, “taRheels#1” on the second change and so on. Or it might be changed to “tarheels#11” on the first change and “tarheels#111” on the second. Another common technique was to substitute a digit to make it “tarheels#2”, “tarheels#3”, and so on.
“The UNC researchers said if people have to change their passwords every 90 days, they tend to use a pattern and they do what we call a transformation,” Cranor explained. “They take their old passwords, they change it in some small way, and they come up with a new password.”
The researchers used the transformations they uncovered to develop algorithms that were able to predict changes with great accuracy. Then they simulated real-world cracking to see how well they performed. In online attacks, in which attackers try to make as many guesses as possible before the targeted network locks them out, the algorithm cracked 17 percent of the accounts in fewer than five attempts. In offline attacks performed on the recovered hashes using superfast computers, 41 percent of the changed passwords were cracked within three seconds.