I think this is going to be hard to answer accurately. Any snake's growth rate will vary based on its individual genetics and feeding schedule. A jcp that's destined to be 7 feet long as an adult will probably be larger at 1 year than one that's going to end up as a 4.5 foot adult. Similarly, a snake that's a willing/aggressive feeder will grow more quickly than one that only eats every other time food is offered.
Someone who breeds jcps might be able to give you a reasonable average number, but realize that individuals are unique and your results will vary at least a little from what the average is.
Mountain bikes are for slow people, and reptiles are far better pets than cats & dogs!