Training happens all year around. So, if you do the math, every 10 weeks, a new cycle starts. Add up all the different companies at a training facility, and then the number of training facilities the Army has. I have not sat down and done the math on this, but you get the general idea that a new cycle is starting every few weeks somewhere.
As to Chemical AIT, in other words 75D, this is also offered yearly. Being 10-11 weeks long, it's likely offered several times a year. If you need more detailed information on what slots are available to you and when, you'll need to talk to a recruiter.