NO NO NO NO.
don't make this mistake ( or read up on pharmacology) ,
For example :
the LSD half life is very short , but the effect is long.
the half life of THC is very long but the effect is shorter.
PING...
While you're right in a general sense, i.e. one can imagine a drug that lasts for a long time with a short half-life, or even more likely, a drug which has a long half-life, but is short acting, you're examples are not correct.
The half-life of LSD is far longer than THC, Aghajanians Naval Study showed it to be 175 min, while Papac's more recent study showed it to be 5 hours, so averaged about 4 hours. Which fits perfectly.
THC has a v. short INITAL PLASMA half-life, esp. after smoking (Indeed, in rats, the plasma half-life of I.V. THC is less than 5 minutes). This is because it is sequestered into tissue very rapidly. The TERMINAL plasma half-life is very long, (up to 57 hours) as the THC is "drip fed" out of the tissue, but this is in levels far below the levels to get you high. i.e. The levels of THC that get you high leave you plasma with a half-life of approx 20 minutes. See review by Grotenhermen.
It is a very fair assumption that a drug with act with a similar time course to its plasma concentration (usually if you plot drug effect vs plasma concentration you get a circle, plasma concentration leads effect on the rising phase, but as the drug wains, the effect outlasts the plasma concentration).
Aghajanian, GK. and Bing, OHL. (1964). Persistence of lysergic acid diethylamide in the plasma of human subjects . Clin. Pharmacol. Ther. 5: 611–4.
Papac DI, Foltz RL (1990). Measurement of lysergic acid diethylamide (LSD) in human plasma by gas chromatography/negative ion chemical ionization mass spectrometry. J Anal Toxicol 14 (3): 189–90
Grotenhermen F. (2003) Pharmacokinetics and Pharmacodynamics of Cannabinoids. Clin Pharmacokinet 42 (4): 327-360