Time of lay and rate of lay are relative to the particular strain of cochin with which you are dealing. Some breeders strains begin lay later 10 months to a year but will lay with more persistency than other breeders birds.
Personally, my large cochins begin to lay at about 8 to 10 months, usually October, and will continue until about June. They tend to be very broody and if you leave eggs in the nest a few days in a row a cochin hen will soon take them over.
Bantam cochins are more enigmatic. Mine tend not to lay until the following spring after hatch which means they are close to a year old when they begin to lay. The older hens are even later, they tend not to lay until the day length gets just right and the weather warms a bit. I have found the bantams to be poorer layers than the large cochins and have shorter durations of lay. Of course, this all depends on where your cochins came from as all breeders breed for differing qualities, start and rate of lay being just a couple of those.
Bo