How far downwind of a covid-19 emitter has a 1000-fold reduction in PPM?
As I said in my comment, viruses aren't my area (concerning units). But the answer is actually unitless. The exact answer you seek is dependent on a couple of different variables:

The speed of the exhalation (cough or breathing)+speed of wind
The atmospheric stability

Of course, the viral load of the exhalation is also important, but since you're asking for a ratio, I won't list that.
The Gaussian Plume model can be used to describe pollutants. The Gaussian Plume model is: $$\frac{C}{Q}=\frac{1}{2\pi U \sigma_y \sigma_z}\exp\left(-\frac{1}{2}\left[\frac{y}{\sigma_y}\right]^2-\frac{1}{2}\left[\frac{z-H}{\sigma_z}\right]^2\right)$$,
where $C$ is the concentration, $Q$ is the emission rate, $U$ is the wind speed (or worst case-scenario is cough speed), $x$ is the downwind distance, $z$ is the distance from the ground, and $\sigma_x, \sigma_z$ are the horizontal and vertical diffusion constants.
$\sigma_y, \sigma_z$ are functions of stability and wind speed. The actual formulas are... iffy. Theoretically they can be described as $$\sigma_{y,z}=\sqrt{\frac{2K_{y,z}x}{U}}$$, where $K$ is the eddy viscosity. There are empirical formulas based on stability classifications too.
Lets assume that the receiver is directly downwind and at the same height (z=H and y=0). Therefore the equation collapses to: $$\frac{C}{Q}=\frac{1}{2\pi U \sigma_y \sigma_z}$$. Since the distance downwind is expressed only by $\sigma_{y,z}$, then to answer your question, we need to isolate $\sigma_{y,z}$. $$\sigma_y \sigma_z=\frac{1000}{2\pi U}$$
Let's just say, for the sake of argument, consider two different scenarios:

Empirical $\sigma_{y,z}$ (refer to this for values of constants)

$$(ax^b)\times (cx^d+f)=acx^{b+d}+afx^b=\frac{1000}{2\pi U}$$
And this is complicated to solve. So I'll leave solving for $x$ as an exercise to the reader.

Constant $K_{y,z}$

$$\sqrt{\frac{2K_{z}x}{U}}\sqrt{\frac{2K_{y}x}{U}}=\frac{1000}{2\pi U}$$
$$\frac{4K_{z}K_{y}x^2}{U^2}=\left(\frac{1000}{2\pi U}\right)^2=\frac{10^6}{4\pi^2U^2}$$
$$x=\sqrt{\frac{10^6 K_{z}K_{y}}{16\pi^2}}$$
$$x=\frac{10^3}{4\pi}\sqrt{K_{z}K_{y}}$$
If we assume that $K_z=K_y=K$, then the equation simplifies further. $$x=\frac{K10^3}{4\pi}$$
There are many ways on how to calculate $K$. One simple formula for $K$ can be used if mixing length theory is invoked
$$K=\frac{ku_*z}{\phi(\frac{z}{L})}$$, where $k$ is the von-Karman constant, $u_*$ is the friction velocity, $\phi_M(\frac{z}{L})$ is the Businger-Dyer function for momentum, and $L$ is the Monin-Obukhov length. If we assume neutral atmospheric stability,$\phi_M(\frac{z}{L})=1$ and the resultant equation can be written as:
$$x=\frac{ku_*z10^3}{4\pi}$$
More assumptions! Let's assume z=1.5 m and $u_*=1.11$ m s$^{-1}$ (corresponds to a 5 kt wind speed at 10 m with a roughness length of 0.1 m under neutral stability). Plugging those numbers in gets
$x=35$ m
Of course, there were a LOT of assumptions to get to this point. But this is what I have to offer.