How long can a sentence be for probabtion violation?

This is all up to the original sentencing judge. It could very well be for whatever time was not served in jail but was in the original sentence. Consult the original sentencing paperwork and it is usually spelled out there in detail.

Anywhere from 6 months to 3 years depending on the offense

Has nothing to do with the offense. It has everything to do with what was in the original sentencing order of the court.