Well, the antibody tests are coming in a matter of weeks - multiple approaches have already been developed and they are currently being validated, that is, checked if they trigger reliably on the Coronavirus antibodies and ONLY on those and not similar ones (source: Charité Berlin - the virology lab there is on the forefront of developing Coronavirus tests and assists in validating new antibody tests).
They will allow us to make large randomized studies in the population and finally determine the "real" percentage of people who can be considered immune, which is the key element currently missing in all projection models. Having that data, more realistic projections can be made as to how fast the virus will resume to spread if we lift specific restrictions. At the moment, we can only hope for this number to be far larger than expected.
But doing such studies takes time, and without their results, lifting any of the restrictions is going to be a dangerous gamble with public health. So I would also suspect for most restrictions to stay in place for about 2 months, and then maybe a slow and gradual opening. First schools, then essential day-to-day businesses, and travel/leisure stuff last.
However, antibody tests are not a substitute for a vaccine. Only a broadly available vaccine will give people enough confidence of not contracting the disease for them to start living a normal life again. We can't expect any kind of normality in the economy until this happens, regardless of whether "America (or any other country) is opened for business" again or not - because an open business is worth nothing if the customers stay away.