is there a rule of thumb that determines when the calipers should be rebuilt?
thanks.

If you're asking about a mileage or time interval for rebuilding the calipers, I don't know if there is one. Here are a few circumstances where rebuilding would be necessary:

1. Brake fluid is leaking from the caliper piston area.

2. The caliper dust boot is torn or cracked.

3. The caliper piston has seized.

Note, however, that a simple rebuild may not solve the problem for any or all of the above cases. If you see signs of corrosion, scratches, or pitting on the piston after disassembling the caliper, then you can't be assured of a tight seal and you'll have to replace the piston, or the entire caliper if new pistons aren't sold separately.