How long for an ASIC cluster to generate most private keys
The private key space is really, really big.  Assume your ASIC cluster can do a billion keys per millisecond, and we'll give you your 140 years.  That will result in around 4.5*10^21 keys, or 4*2^70.

Private keys are 256-bit, so at the end of your 140 years you will have covered about 1/1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 of the keyspace.