This is a bit pedantic, but x64 refers to Alpha, which existed long before 1999. 64 bit x86 (x86-64, or amd64) wasn’t purchasable until 2003, although it was announced in 2000.

There were several additional shifts between 1978 and 2003:

• 8088 / 8086 has what’s essentially bank switched 16 bit addressing which gives 1 MB, or 2^20 bytes
• 80286 has physical support for 16 megs, or 2^24 bytes
• 80386 has physical support 4 gigs, or 2^32 bytes
• Pentium Pro has PAE support for 64 gigs, or 2^36 bytes
• AMD Opteron from 2003 has support for 1024 gigs, or 1 terabyte, or 2^40 bytes
• Current AMD and Intel CPUs physically support anywhere between 2^48 and 2^57 bytes of physical hardware (256 terabytes to 128 petabytes)

But let’s just use three points of data: 8086 / 8088, 80386, and let’s say the first 64 bit AMD Opteron supports 64 bits:

• 8086 / 8088, 1978, 20 bits
• 80386, 1985, 32 bits
• AMD Opteron, 2003, 64 bits

1978 to 1985 is 7 years, with a change in addressing of 12 bits, or about .6 bits per year.

1985 to 2003 is 18 years, with a change in addressing of 32 bits, or about .56 bits per year. So far, pretty consistent.

How long would it take to go from 64 bits to 128 bits? At around .56 bits per year, that’d be about 114 years, and we’ve had twenty so far.

Check back in 94 years.