A 120mg pill of Sudafed (Pseudoephedrine hydrochloride), for example, lasts for 12 hours. You are not supposed to take another Sudafed pill before the 12 hours are up.
How exact is this 12-hour limit? Do i have to wait at least exactly 43200 seconds between pills? How much leeway is there? Could i take the second pill a few minutes (5? 10? 30?) before the time is up?
Note that Sudafed is used as an example here. The same should apply to any other drug with a time limit.