Cops may follow drivers for as long as they wish without pulling them over. They must have probable cause to pull a vehicle over, such as for speeding, a broken tail light or failure to obey a traffic signal.
A police car behind someone does not necessarily mean that the police officer suspects the driver of wrongdoing. It may just happen to be that the police officer is traveling in the same direction. Likewise, cops are not likely to follow someone for a long period of time unless they are confident a tort has occurred and are waiting for a legit reason to pull them over. Some police departments do have protocol stating that officers must not follow drivers beyond a certain distance without stopping them, but this "gray area" is one in which judges most always rule in favor of law enforcement.