Block rules [examples]
These pictures show the behavior of rule (c) starting from some special initial conditions.
The repetition period with a total of n cells can be 3n steps. With random initial conditions, the period is typically up to about 3n/2. Starting with a block of q black cells, the period can get close to this. For n=20, q=17, for example, it is 31,300.
Note that even in rule (b) wraparound phenomena can lead to repetition periods that increase rapidly with n (e.g. 4820 for n=20, q=15), but presumably not exponentially.
In rule (d), the repetition periods can typically be larger than in rule (c): e.g. 803,780 for n=20, q=13.