Analysts essentially calculate this number by taking cumulative chain work and dividing it by the hash rate at that time. Accordingly, an attacker has to mine a different chain. They can then publish their blocks as soon as the chain has greater proof of work than the main network.
Bitcoin security is at an all-time high as it will take an attacker with complete control of the network over 400 days to rewrite the blockchain even if they have 100% control of the network’s hash rate.
A prospective attacker would, therefore, spend over a year and hundreds of millions if not billions to attack the Bitcoin network. After all this work and resources, the price of Bitcoin could still come crashing down. This means that from an economic standpoint, an attack on Bitcoin makes no sense.
For perspective, a one hour attack will set them back $597,128 per hour at the time of writing.