It is possible to dull a bur in seconds if it is run at too high a
speed, so that it overheats, even with lubrication. Slow even
cutting, keeping the lubricant from burning away is the secret.
Having a low speed, high torque setting flex shaft does help in
keeping the speed and heat down, but good control of rhe shaft speed
is the key what ever set up you are using.
I have burs that are several years old and still sharp, while I have
a box of burs that died because I rushed a bit too much. I do not
toss the damaged burs, but keep them to make other tools. small
punches, burnishers, etc.