Adaptive filtering: Optimum filter length and delay

I'm trying to find the optimum filter length for an Adaptive Filtering, using RLS Algorithm.

I'm using this design:

So the "error" signal is the signal without noise (and that's the signal that I want).

If I have $e(n) = d(n)-y(n)$ but $d(n)$ is my desired signal, I need that $e(n) \rightarrow 0$ so I find the optimum filter length (and the delay) using the MSE criterion, but now I have the signal that I want as the error, so I don't know how to find the optimum filter length, because I have NO idea what MSE do I have to get at the output!

Can anyone tell me what should I do?

Thanks!