This is a cross-post of the unanswered question (the given answer turned out to be incorrect) https://math.stackexchange.com/questions/245591/period-of-linear-congruential-generator .
How can you calculate the probability distribution of the period length of a linear congruential generator? This is $X_{n+1} = (aX_n + c) \bmod m$ where $a$ and $c$ are chosen uniformly from $\{0,\dots, m-1\}$ and $m$ is a fixed prime. Take $X_0$ to be an arbitrary integer from $\{0,\dots,m-1\}$.
If it is hard to do exactly, is it possible to give good bounds for the cdf?