When is a cryptosystem or its algorithm (like RSA) considered efficient?
From a theoretical perspective in cryptography, a cryptosystem is efficient when the execution time of the algorithm run by the legitimate user grows as a polynomial of a security parameter $k$, while the execution time for the best algorithm an adversary can run to break the system grows faster than any polynomial of $k$ (some change that to: exponentially with $k$, thus giving a different, less common definition).

This definition does not require to specify what $k$ is, and it's readily proven that changing $k$ to a polynomial in $k$ does not change the outcome of the above test. But often, $k$ is the size of something in bit, like the key, or the signature. In symmetric crypto, it is typically the size of the secret key.

From a practical perspective, the line is not clear. Make it: whatever is acceptable in the context.