At 100,000 new cases of Covid-19 per day, the US will
have 36 million new cases per year. That is about 1/9 of
the population. If herd immunity requires 70% of the
population to catch the disease, that will take 6 years.
If the rate of infection increases to half a million per day, we might
reach herd immunity by the end of 2021. However, at that rate, most
people that get a bad case are likely to die, because most of them
won't be able to get hospital treatment (the hospitals will be
overloaded).
The way to regain a normal life soon is the way that Taiwan, New
Zealand and parts of Australia have used:
jump on the virus hard and
drive the numbers down, then use old-fashioned contact tracing
followed by isolating those who may be infected to mop up the rest.
This method is known to work,
and it can work in just a few months.
It is true that the US is not an island, but that makes less
difference than people think. The number of people that enter the US
without permission is minuscule by Covid-19 standards, and most of
them surrender promptly to federal agents, which makes it possible to
test them and quarantine them.