2560 bits is stronger than 256 bits and the extra cost does not matter for a tool designed to encrypt files. See what I did here? Where do we stop?
128 bits however is humanly achievable (35 years worth of peak Bitcoin mining), and as such perhaps a tad low in really high stakes scenarios (say highly classified stuff).
We could chose something between 128 and 256, but they're nice powers of two, and we tend to like powers of two.
Current bitcoin miner can provide efficiency of 19 000 000 MH/J or 19e12 H/J. If we would assume bruteforcer can perform with the same efficiency, we would need 1.6e16 J/s = 16 000 TW.
Current humanity energy output is something like 17 TW.
With your numbers if humanity dedicated all its energy to brute forcing a 128/bit key for a year it would have a ~0.1% chance of finding it. I guess that's a pretty good argument that no one is going to even try it in the foreseeable future.
See this cousin comment highlighting my error. that out: https://news.ycombinator.com/item?id=34955855
Edited as not AES but you get the point taking into account that AES256 is the standard.