To get back to the original question: it depends on the system you're in I guess.
With the new JAR system, we had an "agenda" to follow (FTO), where each flight was described in advance. At the end of the first series, there was a "pre-solo check" with another instructor. If OK, the next day you would go on a flight with your personal instructor, if first couple of circuits were good, a full-stop followed, he would get out and you'd go solo if you felt you were up to it.
So from the first day of training, provided you were up to the "speed" of training and the weather was fine, you'd have an idea of when you would go solo.