There on probation in the first place for pocession of marijuania and failed a ua for the first time how long of jail time?

He will be there till at least the hearing. Which can be a couple weeks to sometimes a month or even more, 6 and 7 week are not unheard of.

Since he was here because of a drug issue and already shows he is still on drugs the judge may not see a reason to be nice. But most likely if there is more time, a few months, a ordered drug program, and put back on probation. BUT>>> of course the judge can also put him in jail for the entire probation time.