I'm sure somebody else on this forum is way more qualified than I to define the time to replace a pair of custom footbeds or have them assessed for appropriateness by a boot fitter or medical professional. However, here are some of my criteria:
- The rigid part of the footbed cracks - particularly in an area where you can feel it.
- The footbed is too small for your new boots.
- Your weight has changed more than 25 pounds and remained that way.
- Your daily activity has dramatically changed - you change the number of hours per day you are on your feet or what you do while on your feet for a significant portion of the day.
The last two items may take some explanation. Changes to the load your feet are required to bear will change their shape. The impact of a change in weight in that regard is easy to understand - as you gain weight your feet spread out to handle the load and vice versa. If you change from standing all day to walking or working on your feet all day your feet will get more muscular and vice versa. If you change from sitting all day to standing all day your feet will spread out too.
So, I guess the answer is to take your custom footbeds to your boot fitter and ask them if they are still appropriate for you.