About 8-12 hours
8-12 hours is what is referred to as the 'half-life' of this medication.
That means that 1/2 of the drug has been metabolized (removed) in this time period. Although half of the dose is still present after this time period, it is understandably much less effective for every hour beyond this. The maximal (usual) effect of this drug, then, is good for the first 8-12 hours.