If a person is on probation and violates that probation, how much jail time is usually given as a punishment?
First, I have to say that I am not a lawyer. Regardless of that, I think I can give you a fairly accurate answer to your question.
That answer is: it depends. There are lots of variables.
If a person is on probation already, it is because they have done something wrong in the eyes of the law. They may have already spent time in jail, or maybe not, but probation is basically saying "keep your nose clean in the future or we'll be seeing you again." So the first variable is how serious an offence a person was put on probation for in the first place. The nastier it is, the nastier the punishment for violating probation.
Secondly, the nature of the violation that "breaks" parole comes into play. If an individual violates parole in a way that is related to the original offence the punishment will often be more severe. For example, a person who is on probation for fighting who gets into another fight is worse off than someone who violates probation in some non-violent way.
Third, the nature of the individual as gauged by the authorities. What are the reasons for the violation; how repentant is the individual; how honest were they when confronted?
Much rests on the judgement of the probation officer. That individual may choose a course of action ranging from a warning to a probation hearing. Again, it depends a lot on other factors. The hearing may not result in jail time, depending. It is possible to receive extended probation time, additional probation conditions, monetary fines, community service, or a brief stint in jail. In some cases probation can be cancelled and a person sent to jail to serve out a previous sentence.
In short, people are put on probation as a way to avoid having to lock them up for doing something they shouldn't have been doing in the first place. If a person violates that probation, it is calling into question the judgement of the individuals that allowed the person to remain "free" in the first place. It generally shows a lack of judgment and as well as a lack of remorse, and our justice system gives big black strikes to both.