My brother is in jail for probation violation for failing a drug test for the 2nd time and he's on 1st offender for 8 years; how much time will he do?

Impossible to know and foolish to guess. This is also an international site - what Country or State?

He could be sentenced to the full sentence for the original offense plus additional time for offending.

Does he have a Public Defender or private Attorney?

It appears they said GA, ( Georgia) and I will assume the state Georgia not the country.

First they will lose their 1st offender program if violated. That means any agreement for it to be expunged after probation will be gone. I would say that they will first be in jail for 1 to 2 months till the probation hearing, depending on where in GA. Next my best guess, they give him 3 to 6 months in jail, to reflect on how serious his errors have been. And put back on probation.

But anything can happen, he could be sent to prison for the entire sentence of 8 years. ( but I would say that is less likely)

He needs to get a private attorney as this is now a post conviction matter and a public defender is not appointed in such matters. He needs to remember he just blew his first offender program out the window. He could possibly be sentenced to his original jail sentence. Get out the original sentencing paperwork and look at that for your answers.