Shimano 6800 bottom bracket life
 

What have you seen in mileage to when it required replacing ?

There is no one number for your question. How heavy are you? Do you ride in bad weather routinely? What type of riding do you do, road, gravel, etc.? I have over 20,000 miles on each of two BB-6700 bottom brackets (the 10-speed predecessor to the 6800) and they are still in fine condition.

This is the answer. There is no definitive number of miles or kilometres that any bottom bracket will last

Also a major variable is how perfectly aligned the bearings are, which is up to the facing of the BB shell for a threaded BB, and the machining of the BB for a PF one.

Run it until it feels bad. If this is less than 4k miles or so and you haven't been running it in bad conditions, maybe check out the BB shell or crank preload.

You can also re-grease your bearings to make them last nearly forever. Just use a thin blade to pry off the seal, drop the BB in some solvent or spray with solvent to remove old grease, let dry, pack with fresh grease, replace the seals, install BB with grease on the threads to prevent corrosion and you're done.

My Habby Ti has 7500 miles on the 6800 BB and 2 days ago I clean the bike, use quick link to take of chain to clean, the BB spins like a top. In perfect shape so I would guess easily 20000. The real thing is that I have avoid all bikes with press fit BB and unless forced by none being made, never will buy anything except standard BSA threaded BB. I have 15000 miles my the 6700 group BB still spin like tops too.