When will my child understand rules and discipline?

Depends on rules: For younger children especially, rules need to be firm and easy-to-understand, and always enforced if you expect to teach it. A good rule: "no hitting" (you know exactly when that rule is broken, and can enforce it.) a vague, hard to enforce rule like "don't squirm at the table" is going to be difficult to define and enforce, so it won't work on a young child.

Up to you.: The concept of "no" comes into a child's head when whatever they want to do, that the parent says "no" to, becomes physically impossible to do. Holding your child gently but firmly while saying "no" will get the message through the quickest.

Learned behavior: Starts at 6 mos., when babies are smarter than puppy dogs. Use a low, firm, "no" & redirect baby consistently. Neurotypical babies & toddlers understand ~ 100 x more than they say, & they say 3 words at a year. Discipline is teaching, not punishment. A 16 mo. Old will do what you do, not do what you say. At 18 mos. You can use time-out. Effectively. See " little people" by ed christopherson.