How long does"24-hour" loratidine (i.E. Claritin) really stay with you? Maybe 16 hours, you think?

Great question! Loratidine is metabolized to it's active metabolite, desloratidine which has a half life of 28 hours (range 9-96). This is very long acting but there is variability among individuals based on how your liver metabolizes it which is genetically determined. If you take a drug for 5 half lives, you develop a steady state blood level ie it's more effective if you take it every day than just occ doses.