It varies greatly with each individual. I had my preexposure shots about 4 years ago when doing wildlife research and since then (due to my job) have had my titers checked every 2 years, and my Ab levels are till pretty high. A man I work with had his shots 12 years ago and he is still good to go with his titers too. Some people may not last that long though and some titers go down after even 4-5 years, so it just depends, but normally they stay within the acceptable range for pretty good amount of time.
BTW brand new to posting here, but have been a regular visitor for awhile and decided to join the crowd 🙂