The length of training depends on the complexity of the product(s).
In most cases, an initial product training of 3 to 4 days, plus a one-day training on tools and processes, is enough to cover all the basics so that the operators can provide clear answers to your customers.
Then, you should consider continuous training. For example, scheduling a half-day session every six months is helpful to refresh processes and review new information in the knowledge base.
The smoother the communication between us, the better the quality of the responses.