Ok, we'll there is good news, and there is bad news. I'll begin with the bad.
1) How quickly does an arrest show up on a background?
An arrest can show up within 24 hours, depending on the type of search the employer conducts. There are a few places where an arrest record can/will appear, which includes the local clerk of court’s database. Also, an employer can perform their own criminal background check by checking the database to see if you have an open case and a recent arrest date. Employers can also view the booking records of the local jail. They can call the sheriff’s office, police department, probation office, or another law enforcement office directly to ask for information.
The good thing is arrests are generally not given much weight compared to convictions. This is because anyone can be arrested for anything and does not, under the law, prove that you committed the alleged offense.
2) An arrest, alone, will generally not exclude you from employment
This goes to what I had mentioned before. The reason, again, is because most employers know that many people are arrested and evidence later shows that you are not "guilty" of the offense (for any number of reasons). That is why the overwhelming majority of job applications ask "If you have ever been CONVICTED of a criminal offense", not arrested.