Most drug cases are relatively straightforward; the police find drugs on you, or in your car or house, and you get charged with possession. If you have a large quantity, you might get charged with possession with intent to distribute. Occasionally though there may be a lengthy investigation.
The police may decide to investigate further in several situations. Sometimes a person will be arrested with a small amount of drugs. Instead of making an arrest for a minor offense, they may offer the individual a chance to "work his case out". That involves providing them with information on where he's getting his drugs from. They may even use the person to make a controlled buy, under police surveillance. They might then arrest that person and continue doing the same thing, working their way up the chain.
If they are going to file charges, they may wait until they've gone as far as they think they can go.
When the police suspect an organization is involved they might try to put together a conspiracy case. Usually those are federal investigations, since it is far easier to establish a conspiracy in federal court than it is in state court. It's not uncommon for those investigations to last months, and sometimes longer.
Sometimes people will come talk to me after they've been questioned, or suspect they may be the target of an investigation. There are a number of questions people have, such as should I cooperate, and do I need a lawyer. As you might guess, my advice is to get a lawyer sooner rather than later. The obvious reason is that you don't want to do or say anything that will help the State make a case against you. Another reason is to get some advice about how much trouble you may be in. Most lawyers who have experience in drug cases can figure that out.
There's still another reason to get a lawyer early. Sometimes people think they can talk their way out of the situation; all they have to do is explain things. Trust me - that rarely works. Instead, you usually end up digging a hole for yourself. There may be information the police need to know though so they will have a complete picture. A lawyer will know the best way to present it - or even if it's something you want to present.
The worst thing you can do in this situation is sit back and hope you don't get arrested. By then it's too late - and you really do need a lawyer.