Being arrested comes with a lot of uncertainty, which can include not knowing the exact charges being filed against you. The authorities can't wait forever to inform you of the charges, but they also don't have to tell you immediately.
For suspects who are in custody (jail), some kind of court appearance (usually an "arraignment" or "initial appearance") must typically occur within two or three days. At that court appearance, the government presents the charges against the defendant. The charges are normally subject to change, though. For example, a prosecutor might not make a final decision on charges until after a preliminary hearing, which can take place more than a month after arrest.