For example AES-128 starting with a 128-bit message $m_0$ and static 128 key $k$
continue until $m_i$ is equal to any $m_j, j<i$
The period length would be $l = i-j$
Any theory about how big that $l$ will be?
Will it be equal for any possible $m_0$?
Is $l=j$ for every $m_0$?
(edit: in AES $j$ is always 0 because symmetric algorithm. Each cipher value has only one possible plain text)