That's impossible to answer. It depends on the complexity of the case, when the hearing starts, how many witnesses the prosecution will call, the detail of the cross-examination by defense counsel, how many defendants, etc.. It could last an hour or 4 days. You should call your brother's attorney to get his/her estimate.
A PH in a murder case could take a week or even a month. It is not something you can take in by missing a little time from school. If this is the first setting of the PH it will probably be continued. Murder cases move very slowly through the system.
The point of a preliminary hearing is to attack the credibility of the officers and witnesses and tear the DA's case apart. Attorneys do not wish to rush them. How long they take depends on the facts of the case, and how many officers, witnesses, etc. there are. It also depends on what, if any, witnesses your brother's attorney wishes to put on the stand. You could ask your brother's attorney how long it will take. And while I am sure your brother will appreciate the support, do no do anything that will hurt your schooling or your future. Good luck to your brother!
The above stated is advice only, and does not establish an attorney-client relationship.