If US emissions went to zero how long until global emissions reach current levels
That will depend on the emission scenario you pick for the future, the more recent scenarios are the RCP's (Representative Concentration Pathway).

RCP scenarios

The scenarios involve future pathways in the emissions of a large range of different chemicals (Greenhouse gases and aerosols). Each RCP have a temperature tag, for example RCP6.0, is a scenario that consider many geopolitical, social and economical predictions and that would result in an emission pathway that would the radiative forcing (input energy to Earth) by 6.0 W/m$^2$ by year 2100.

The data associated with the RCPs can be accessed in the RCP Database, this is a plot generated there with Global $\text{CO}_2$ emissions for all four RCP scenarios.



According to this data, the U.S. was responsible for 14.34% of the global emissions of $\text{CO}_2$ on 2015.

If we consider that overnight all scenarios switch to a  version of themselves with a 14.34% emissions reductions (i.e. like if the US stopped all its emissions suddenly), the plot would look like this:



Where the dashed line is the new emission pathway and the thin dotted line marks the global emission level before the US $\text{CO}_2$ shutdown.

We can see that for RCP2.6 and RCP4.5, the world would never get back to emission rates as high as those of today. And for the more pessimistic scenarios it would take at least 10 years to go back to those levels: For RCP6.0 it would go back to current levels by 2041 and for RCP8.5 by 2029.

There are many simplifications on this calculation, but gives you a first approach. If you are interested in another emission scenario you can apply the same methodology or a better one, but maybe this can serve as a starting pint.