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.