This is just from my own experience, but it seems to work quite well.
Someone just starting out is going to need a bit more time to rest up between sessions, since the new student is going to be using muscles that haven't really been put to the test before. As a general rule of thumb, for the first couple of months, twice a week, one hour a day, is a good start.
As the new student gets better conditioned, and more used to the regimen, that's when the amount of training can be upped to maybe three days a week, one hour a day. Once the student has had about six months of training, that's when upping things to maybe two classes a day for some of those days, can really help accelerate things.
Once the student has been in there for a year, it may be entirely possible that training every day can be possible, and beneficial, although this has to be evaluated from person to person.