Expected Linear Complexity of a sequence
It means (usually) averaged over all sequences of a given length. Rueppel computes this in his book.

It's for an i.i.d. uniform sequence, Proposition 4.6. As $n$ goes to infinity by the law of large numbers this sample mean approaches the actual mean.

This was rigorously proved by Meidl and Niederreiter much later. Behind a paywall but you can see the abstract below:

https://ieeexplore.ieee.org/abstract/document/1042274/

The answer is nearly $n$ for $n$-periodic sequences.