They are able to reach their sexual maturity when about 7-12 feet or in as little as 18 months for males. Females however take much longer and are often unwilling to breed until they reach the age of 4 or 5. The important thing is not to assume that because your retic looks big enough that it is ready. This will only lead to heartache on your part.
I have listed below the two main directions people take in order to get their retics to breed:
Stop all feeding a minimum of 2 weeks before this begins in order to get any previously undigested food out of the snake. Animals should have excellent weight and be established before any breeding is attempted. Retics can be cooled in the low 70's during autumn. This temperature drop is to be achieved gradually in order to reduce the stress on the snakes body and to avoid any respiratory infections. During this period it is an ideal time to reduce the daylight time they receive to around 9 hours in order to simulate natural winter daylight conditions. They stay in this state for 4-5 weeks before the next step.
Then you place the male & females together at the cooler temperatures and mist the cage for a couple of weeks. This is done by adding the female to the males cage, this simulates the female entering the males territory as she looks for a mate. There must never be another male in the same cage as they can and will fight causing injuries to each other that often lead to death. Please be aware that males are often more aggressive during this period.
There is a more recent thought that retics do not require cooling. After all these are tropical snakes that originate from near the equator. Their wild counterparts are not subjected to such dramatic temperature variations so there should be no need for captive animals to either. While the first method listed can be seen as an attempt to bring the females into season this method relies on the good husbandry and the health of the female. Every year healthy females will stop feeding for a short period, at this point near by males can be seen to start cruising around their vivs. This is a sign she is ready. It is then you should introduce the male, as the female will now be receptive.
A few common actions females undertake when males are introduced are often mistaken as signs the female is ready to breed. The most common by far is the tail wagging and musking. This happens when a male tries to mate the female. She will wave her tail from side to side in the air, if after repeated attempts the male still doesn’t understand she will musk the area at the same time, creating a shower of musk all over the area. If that happens the male usually gets the point and they both settle down again.