Class action lawsuits typically take a long time, often several years. First, a great deal of detailed medical information needs to be collected and presented, and interpreting that information takes a lot of time.
The stakes are also typically very high, and in cases where a drug company is found guilty, total damages awarded for the entire class together often are measured in hundreds of millions or billions of dollars. With so much at stake, both sides in the suit have an incentive to fight as long and as hard as they can.
This means that even after the initial ruling is made by the court, there may be several more years to go as either side can appeal the ruling. Once a final settlement is reached (in cases where the drug company is ultimately found guilty), there are still often substantial delays depending on documents being prepared and issued to all members of the class.
In fact, in addition to efforts to notify all potential members of the class at the start of the suit, there are often extensive advertising campaigns to make sure sure anyone who could be a member of the class knows about the possibility of receiving a portion of the award decided by the court.