2. 12 hour Sudafed has 120mg of pseudoephedrine released across a time span that will have lasting effects of 12 hours. Regular immediate release Sudafed tablets have 30mg, but you can take 1 or 2 tablets and the effect last about 4 to 6 hours. So if you cut the 12 hour tablet, even though you destroy the extended release mechanism, it still will be the same as taking 2 tablets of regular Sudafed (60mg) which will last about 6 hours.