A lot of the pharmacokinetics have to do with half lives. If it has a short half live generally it will be dosed more often and vice versa. Saying that there are a bunch of small things that also change it. You have to factor in if it is hydrophobic/hydrophilic or lipophobic/lipophilic. Does it leave the blood stream or not. Does the liver or kidney metabolize or remove it. Does it cross into the brain. And more...

Someone else mentioned phase 1 trials. That is where they do a lot of this. Give a dose check blood levels every 30 min and check the level.

Saying that they can make ER (extended release) or DR (delayed release) versions that release over time so it can be taken less often.

Some antibiotics have different types. Some have to keep the blood concentration over a certain level the whole time. Some only need to go over a certain level for a certain length of time every so often.

Also, specifically for OTC meds often times the OTC dose is not the max but the max is set when it is approved for OTC by the FDA. For instance ibuprofen will have max dosing lower than the prescription strength. Omeprazole will say only to use for 14 days before you see a Dr. There are legal or safety reasons for that.