In some cases, the police will arrest activists, transport them away from the scene of the protest (sometimes many miles away), get personal information, and then release them onto the street without charge. This might occur if the police are unsure if any crime was really committed and just wanted to clear the area. Prosecutors may then be consulted to see what, if any, charges will be filed. The prosecutors can then take anywhere from a few days up to a year to decide to file summary charges against you, or even longer for some more serious (indictable) charges.
If this happens to you, give a good mailing address to the police. If police decide to charge you later, they may send a summons to the address you have given. The court where the charge has been filed will also send notice of your court date to the address you give the police. If the notice is mailed to a bad address, it will be returned and you won't get notice of the court date. If you don't attend court, the case could be heard in your absence or a warrant for your arrest may be issued.