It is well known that Clenbutaplex (Clenbuterol) use results in rapid down-regulation of beta 2 receptors. This is due to the powerful stimulatory effect of the drug. It is therefore pointless to use clen for long periods without a break. Some believe that a two day on, two day off dosing schedule will allow adequate potential for receptor up-regulation. However, I doubt this to be the case due to the relatively long half life of clen, resulting in continued stimulation even throughout the 'off' days. A much better regime would be a two week on, two week off cycle. Maximum plasma levels are reached around 2-3 hours after oral administration, and terminal half life at 34 hours (Zimmer, 1976).
Also, bear in mind that clenbuterol has some side effects. It isn´t great for your heart, and can cause some issues there (enlargement of ventricles, etc, ) but most studies showing clen to cause heart problems are with animals, and even though the dosing is almost similar to what humans take (in some studies it´s within the range of what would be double of a large human dose...) it´s important to remember that animals have more beta-2 receptors and they cause certain event chains that humans´ beta-2 receptors may not. Clen causes cardiac hypertrophy and cardiac necrosis (cell death) to some degree, in some cases. Again though, many studies showing the more significant, possibly irreversible, heart problems are with mg dosing. We humans take clen in mcg doses.