How do I get the equivalent strength of an ECC key?
The security level of an elliptic curve group is approximately $\log_2{0.886\sqrt{2^n}}$. You can use this to approximate the security level of a $n$-bit key, eg:

$\log_2{0.886\sqrt{2^{571}}} = 285.32537860389294$

The real computation (at least for curves over a finite field defined by a prime $p$) is $ \log_2{\sqrt{\pi/4}\sqrt{ℓ}} $, where $ℓ$ is the order (number of points) of the subgroup of points on the curve generated by the public base point. The size of $p$ determines the upper bound of $ℓ$, though it is not exactly the upper bound itself, it's generally pretty close. So for example, Curve25519 has an $ℓ$ of about $2^{252}$, so:

$ \log_2{\sqrt{\pi/4}\sqrt{2^{252}}} = 125.82537860389293 $

Our approximation based on the ECC key size gets us pretty close to this:

$\log_2{0.886\sqrt{2^{256}}} = 127.82537860389293 $

As otus notes below, Koblitz curves are slightly different.

The strength of any ECC key is bound to the strength of the underlying curve and domain parameters, and in general all the valid keypairs generated for a specific curve should have the same size/strength. There are some points that are invalid and should not be used, but your ECC implementation should handle that for you.

More info on determining the runtime of Pollard's Rho and thus the theoretical security level of various curves at SafeCurves: http://safecurves.cr.yp.to/rho.html