Most places there's a 24 hour wait before they act on a missing person, but for a minor or someone who may be in danger they can get involved immediately. If you are a minor the police must by law bring you back to your parent, guardian, or Child Protective Services.
If you are reported as missing, they will keep an eye out. It matters what other things are going on regarding how long until they start to search. But I don't think it's a good idea for anyone to run away, if that was your plan.