Put it this way: the ester with the shorter half-life must be injected more often if the goal is to have fairly steady serum testosterone levels. If you inject T propionate only every four days then you'll have crashed your testosterone prior to each injection.
With daily injections, the only reason to use a short ester such as propionate is if some amount of serum testosterone variation is desired. I posted above suggesting that even propionate may not offer much variation, but since then I've acquired many measurements while using it and have also seen some measurements from others. These indicate that quite large daily variations are possible with propionate, up to at least +/-50% about the mean, more than double what is physiological. Because of this my daily injection is about half propionate and half enanthate.
Keep in mind that we are using bioidentical testosterone. If you are injecting testosterone cypionate then once the drug is absorbed the cypionate ester is cleaved rapidly, leaving testosterone that is indistinguishable from natural testosterone. The half-life of pure testosterone in serum is quite short, with a range of 10-100 minutes given in the literature. What I keep emphasizing is that with testosterone esters it is the absorption phase that is slow, with half-lives on the order of days.
